"So ring the bells that still can ring
Forget your perfect offering
There is a crack in everything
That's how the light gets in."
. . . . . . Leonard Cohen
Bad things breed in a vacuum. The family that doesn't fill their young girls with compliments and acknowledgments is asking for trouble. That includes compliments on their looks. Those self-hatred demons are right there waiting to get at your children. If you don't fill them with pride and confidence, they will fall prey to destructive influences, like following unrealistic ideals of thinness.
In primitive cultures there were rituals that people went through to prove they were grown up and marriageable. One of the positive things about these rituals, I believe, is that they stopped people from obsessing on their appearances to an unhealthy degree. You went through XYZ ceremony, and that's that, you were finished and ready to find a mate. The girls knew they were attractive and the guys knew they were manly men.
One way to combat the absence of any such markers in our society is to dispense compliments and affirmations to people around us, especially younger people. Humans are social beings and they need certain inputs from without to stop going in unhealthy directions.
If your wife or girlfriend has an eating disorder, it might seem like a totally internal thing you can't influence. While I believe that she will have to take the primary responsibility, you might try the following: Tell her she's beautiful every day, and tell her what is beautiful about her. If she hates her body for no reason, tell her regularly that her body is beautiful and what is beautiful about it. Tell her regularly, "You are a beautiful woman." Compliment her on her non-physical attributes and accomplishments every day. Tell her you love her every day. Bring her flowers once a week. Stand back and see if it doesn't start to do something. It could take a while. And in case you are wondering if that is the recipe for making a woman fall in love with you and having a very happy wife or girlfriend: Yes it is.