How do I help a friend who I think is aneroxic?

My friend hasn't been acting herself lately. She's been wearing baggy clothes and never eats anymore. She used to eat all the time. Not to mention the"Fits"that she throws when we try to get her to eat. Her sister claims that she goes to the bathroom often after she eats. She's been moody, and agressive with us, when we just want to help. She blaims it on the fact that she has to live with her grandma for the rest of her life while she's under age because her parents aren't fit to have custidy of her.That doesn't explain why she's always exercising now. She was skinny before she started now she's just a twig and losing a lot of energy. I know she says that she doesn't need help, but I'm worried.

approch her grandmother first and when u approch her have information on how she can get help because she won't admit that she has an eating disorder