Anorexia

Please help. My friend just told me she threw up. On purpose. She has always showed signs of being anorexic. She constantly calls herself fat and almost eats nothing. She is very skinny already. I have told her many times that she isn’t fat and she denies it. She has compared herself to me saying I’m so skinny and she’s fat. I’m a normal size for 14 and I’ve told her I weigh more than her. She just told me it was all muscles. I have tried telling her everything and it ends with her getting angry. It doesn’t help that one of the boys in our grade calls her fat. He does it ironically. He makes fun of everyone and does it just to see her reaction. My friend tends to dramatically react and I know he doesn’t mean it. Anything I can say or do? Also we can’t get away from the boy. My grade only has 12 kids (we are a very small school) and we go to every class together. I know I should tell an adult but I promised her I wouldn’t and she would be embarrassed and humiliated. She is going through some other stuff right now.