What do I do if my girlfriend is developing an eating disorder?

John asked:


My girlfriend is getting a eating disorder, she is only eating a cracker a day and keeps worrying about her own weight. If she even eats a bit more she will throw up afterwards. She has perfectly healthy weight (at the moment) for a girl her age. I am real worried about her getting anorexia later on. She will not listen when I ask her to eat normally, no matter what I say. What could I do as her boyfriend to help her?