I know it's hard, I've been in a similiar situation myself. There are really only two things you can do.
The first is to tell someone who can help her. If you have a close enough relationship, ask her about it herself, and encourage her to seek help, or at least tell her parents.
The second is to continue being a great friend. The situation is hard enough for your friend without losing a friend, always be there for her, talk to her, give her support and all that friendship stuff.
The thing you have to remember is not to think you can help her on your own. I tried and it almost crushed me, but luckily I noticed before it was too late.