When is skinny, too skinny?

A girl I know is super skinny and it's starting to really worry me. She's always been thin but now you can see every bone in her body. When I tell her she is too skinny she says she knows and that she eats all the time but can't gain weight. I don't believe it is an eating disorder because she doesn't like when people tell her how skinny she is. My concern is that it is drug related, even though she denies this. How do you know if its drugs, eating disorder, or health problems? How do you know when skinny is too skinny?