It's lovely that you want to help your friend, but she really needs to help herself - otherwise she'll just end up becoming dependent on you. Sometimes it's too hard to do this stuff. Sometimes people get very attached to their conditions - I don't mean anyone wants to be depressed, just that it can be a kind of self-reinforcing thing.

Nobody wants to be depressed. People who want attention usually need it. Teenage angst is a made-up phrase designed to trivialise genuine feelings of anguish. And all you can really do is just be there for your friend - if she doesn't want to help herself, you can't make her.