If she is making herself sick pretty soon her periods will stop, she will start losing her hair in spots, her teeth will start decaying because of the acid that is in vomit. This happened to my friend a few years ago and my other friends daughter is doing it now. She wont listen to anyone, shes less than 8 stone but still thinks shes overweight.
Try to encourage her to get help before its too late, if you have to speak to a teacher you can trust about it because maybe her parents dont know what shes doing.
What We Do In Life Echoes In Eternity