I know that there IS a book that's called something like "The Body Book" or "The Body Bible" (perhaps), which tells you all sorts of different things you're desperate to know when you're that age.
Like yourself, though, I suffered from bulimia for several years, and I knew all about healthy eating and what my body needed for me to survive... But I wasn't interested in the facts. So, I don't really know what to suggest. I hope she snaps out of it and starts eating healthily. Maybe she's being bullied in school? I know that was a large factor in why I stopped eating. x