If you read that title and thought "what the f*ck is wrong with this woman, of course my friend needs to leave his sorry ass" then you are a kind, compassionate, and powerful woman. That makes you the exact person your friend needs and also crucially important that you do not just tell her "you need to leave him!" I'd like to go over why saying that is a bad idea, and what to say instead.
In case you're wondering, I am a volunteer Domestic Violence victim advocate (along with a Stalking VA and Sexual Assault VA). I have been for about four years now. Before having kids, I volunteered 20+ hours a week directly with victims (now that I have kids I can't give quite so much). This involves a hefty training program, annual continuing education classes, and a certain number of supervised hours, and that's after a psych eval.
So, if you see your friend is being abused, it is (usually) a bad idea to say "He is an asshole. You are being abused. You need to leave him!" The phrase "you need to leave him" is the slightly kinder sister of "why didn't you just leave?" and is almost as damaging. The reason why is mostly the combination of these two things:
The highest risk of homicide for a DV victim is as they are trying to leave the relationship.
On average, a DV victim will attempt to leave 7 times before they actually stay gone.
If your friend leaves in a half-way planned out hurry (risking her life in the process), then your friend is more likely to return back. And then try to do it all over again, risking her life again. Any abuse she avoided during the time she was going will be accrued, with interest, and her abuser will gladly cash her out.
As a DV VA, we are focused on helping a victim who wants to leave, leave, and to help them do it just once. We want that one attempt to leave the relationship to be successful, safe, and final.
Another factor is your friend may not be ready to leave. This can be hard to hear, but your friend may desire to stay in an abusive relationship. She may still believe she loves him. When I'm screening victims, one of the things I'm looking for is "readiness." If she's saying things like "could therapy help him?", then she is likely not ready. If you rush someone who isn't ready, then she's going to rubber band back.
Additionally, do not call him an asshole, a piece of shit, complete trash, or any other completely accurate phrase. She may still love him. She may still think things can work out. She may think these things secretly. If you say that the person she has centered her life around (and if she is being abused, she has indeed centered her life around him) is subpar, you're alienating yourself from her and playing right into the abuser's hands.
So then, the logical next question is what the fuck am I supposed to do then? Just sit back and see her with a new injury every time we hang out?
To answer this, I'd like you to first check out the Duluth Model here: https://www.theduluthmodel.org/wheels/
If your partner has just one or two of those pie slices, they may not abusive, they're just an asshole. It's possible to be in an unhealthy relationship that isn't abusive.
But notice that there is one piece of that pie that directly relates to you, the friend -- Isolation. In my opinion, the pie slices should not be equal, and isolation is the most powerful piece of the pie. And you are centered perfectly to fight it, because you are what the abuser is hoping to isolate her from.
What you need to do is going to sound so stupidly small, but I promise that it's the most powerful thing that can be done to help a victim. You need to get her to truly believe that you have her back, no matter what.
She needs to believe this in her bones. She needs to know this more than she knows the sky is blue. She needs to know that if she calls you a homewrecker and doesn't speak to you for a decade, that she can then call you up, say "I'm leaving him. Can I stay at your place?" and that you will say "Yes, do you still drink white?"
As long as she knows that someone has her back, that someone truly wants her to have what she wants, then she has the little string that will lead her out of the minotaur's labyrinth.
This is where "don't call him an asshole" comes into play. If you call the person she loves an asshole, it's easy for that asshole to tell her "look, see? Your so-called friend is just trying to break us up." And he'll say it so sweetly. And he'll give her a foot massage and order her favorite food.
What? An abuser being sweet and kind? They are all sweet and kind, sometimes. Every one of them. No woman intentionally starts a relationship with an abuser. He was sweet and kind. Then he wasn't. But he will go back to sweet and kind again and then back to abusive again.
Here, have another circle: https://www.center4research.org/the-cycle-of-domestic-violence/ (scroll down past the Duluth Model)
I heard the best analogy for this -- a slot machine. Your friend is sitting at a slot machine, waiting for the jackpot. That machine has given out $5 or $10 winnings to her a few times before. But, usually, she puts in $1 and gets 0 back. Over and over again. It really is the same circuitry in her brain that is making her stay, just like people stay in front of a slot machine and give and give and give and don't get back, just for the chance of the jackpot. It's because, every once in a while, the jackpot gives the gambler just enough to make them stay.
If you call him an asshole, he's going to become the sweetest guy in the world. He's going to tell her "She just wants to end us." Your friend will see "Oh, yes, this is the sweet man I signed up for! And my friend really is trying to get him gone. I just need more patience in his bad times, maybe some therapy, that's all." and will stay. She will then stop talking to you (because you're trying to ruin their marriage, after all, duh) and you will have successfully helped the abuser further isolate his victim. He'll then switch back into abuser mode.
Similarly, don't tell her that she's being abused. She may (likely) not realize it. Just like someone might feel sick, but the doctor saying "it's malignant" comes as a life altering shock, telling a victim that she is being abused can be a traumatic moment in her life. Are you a therapist? Are you ready to help her with that pandora's box you just opened? And then, what happens next? You just dropped this knowledge bomb in her lap and then offered no solutions except "completely up-end your entire life by leaving him, merely based on something a friend said."
So what am I supposed to say, then?
"I am here for you. I am here for you no matter what. You could kill my dog, and I would still be here for you. No, seriously. Look at me. I am deadly serious. I am here for you. I will help you through whatever life throws at you."
Something in that family is good.
What if she point blank asks if she's being abused? Yeah, go ahead and say "Yes, I think you're being abused." At that point, she already knows. It's not the giant knowledge bomb if she's already been thinking it.
It's very important that the victim be at the center of the planning on what to do from this point forward. I know that you know what is best for her. And I know you want to encourage her and push her into doing what is best for her. But (and you're not going to like to hear this), that is coercion and that is not consensual. Ouch, right? But it's true. If she leaves because you pushed her into doing it, then it is against her will. And her will is really, really weak right now, as it is being bombarded by the abuser. It's actually very easy to accidentally push a victim into doing something they don't want to do, like leaving.
Listen to her. What does she actually want to do? Help support her in that.
If she wants to leave, how can I help her? If she doesn't want to leave, how can I help her?
The answers to both of these two questions are the same. She needs to speak with a local, trained DV VA to make a "safety plan." A safety plan can include a plan for leaving. If she's still saying stuff like "he's actually really kind, he just has these moments, and I just need some help on what to do when he's in one of his moods", then a safety plan can also help. It's a plan to keep her safer following whatever she wants to do.
There's a national DV hotline (it's the one I used actually), but I suggest calling the local DV resource center first. We know things. We know which Judge is a softie and what his schedule on the bench this week is. We know if the shelter is full. We know the attorney that takes two pro bono cases a month. We know the movers that will help her move out for free. We know things. The national hotline cannot help in that way.
But I want to do more
Again, I cannot express how, by being utterly loyal to her, you are doing the most powerful thing to help her out of us. But, if you want to do more than that...
Get a guest room ready. Get ready for any pets she may have. Inflatable beds are not compatible with cats, so get a cot.
Set aside some money just for her when she leaves.
Ask if she will share her safety plan with you, and if you can be apart of any of it.
Offer your phone for her to use if her calls are being monitored.
I hope this helps. :)
EDIT: Let's talk about the damn dog. The reference to a victim thinking she killed your dog is from real life. A victim was dog sitting a friend's dog at the victim's place. The abuser ended up beating the dog. The abuser gaslit the victim into thinking that it was her fault the dog was dead. The "logic" of the gaslighting was something along the lines of "You know I hate dogs. If you hadn't of brought that dog over, this wouldn't have happened." This is the same kind of logic that abusers use all the time. "You know I had a bad day at work, so it's your fault I beat you." In this situation, the victim truly felt that she killed the dog (after all, she's the reason the dog was there) and, thus, refused to reach out to one of her closest friends for help in escaping the relationship.
This is why it's important that if you want to help your victim friend, that they know that even if they killed your dog, you'd be there. In order to get help from someone, she needs to believe that deeply in the support from that person. You have no idea what kind of stuff the abuser is going to trick the victim into thinking she did. You have no idea the kind of depraved things that the abuser is going to make the victim think was actually her idea the whole time.
If your level of help involves exceptions, then you are not the best person to help. That's totally fine. We live in a society that pushes us to help all the issues all the time and that's not possible nor is it healthy to try. The victim will know that there are exceptions and will be wary at leaning on you at best. The abuser may or may not know there are exceptions, but if he does, he'll play on that to introduce more isolation. It really doesn't matter if the exception even applies, because even an exception or two gives the victim the idea of "I'll only help you if you're the perfect victim." And there is no perfect victim, even your friend.
If you're curious, I fell prey to this. My abuser would hover behind me while talking with friends and if they said something he didn't like (politically, a bad joke, whatever), he would motion for me to tell the person off. I said awful horrible things to appease him. Burning bridges of long, childhood friendships in order to get another hour or two of security that I would not be hurt. After I left, many of those people forgave me. They'd say things like "Eh, I knew something was going on cause that's not you. Don't worry about it!" So, the things I did that I thought were unforgivable ended up not being. But that doesn't matter because at the time of the abuse I didn't believe that my friends had my back to that extent. I thought there were exceptions.
If your level of support has exceptions, you are not the best person for your friend to reach out to.