Tell an adult who will be helpful! Just this March I learned of a friend who had died in Dec. She started her anorexa at age 17 (we were in high school) and it finally killed her. Her parents were involved in knowing but am not sure how much family counseling happened- it would've been 1975 when this started. I last saw her about 8 years ago and she looked like crap (but did not present as though she felt bad). She was very much a "winner" and had learned the Russian language and been to Russia. Being over-achievers is a trait of eating disorders. They are trying to "please" the family. (Correct me if I'm wrong those in the know with current info- that was what I last understood).
This is a FAMILY PROBLEM!! You will feel better once you know the family is aware and the right resources are being used. And by the way, there is no cut and dry "treatment". But she may need medical attention if not now, then soon, which would lead to getting help emotionally.
You are a good "not really a friend"!!!