Youre too young to have to deal with stuff like that. Thats coming from someone who was in a very similar situation to the girl. It sounds harsh, but why ruin two peoples youth? She needs serious medical help. Bulimia is dangerous and often undiagnosed because you dont get to be really skinny like someone with anorexia. You might want to let her parents in on her eating disorder. She'll hate you for it, but it might save her in the long run.