Anorexic friend???????

My best friend is relapsing into anorexia, and i dont know what to do. i try talking to her but i just camt convince her. She's completely thin and muscular (shes a ballerina...) but she wants that kind of sickly, skinny body. I don't know what to do, or how to comfort her. I'm not going to tell on her to anyone....suggestions?