She sounds like those girls on anorexia tumblers bragging to each other how much they starve themselves. It's just another form of self-harm, and a way for girls to have a little bit of power over their bodies in a world where they are systematically denied power of anything, including their bodies. Not to mention, since it's breasts in particular, flattening them seems like a way to become invisible to sexual predation, to escape it, so 'de-sex' yourself and flee male sexual terrorism, porn, etc. Starvation will also reduce one's weight to the extent that breasts can nearly disappear. Funny how many parallels there are between girls 10, 20 years ago starving themselves, and girls now identifying out of girlhood and binding and taking drugs. It's all, I think, a reaction to the same thing: the horrible realization as girls come of age of what male supremacy has in store for them.