Sometimes, the best way to be a good friend is to risk losing the friendship completely. It sounds crazy but it's true. If you want to do the right thing for your friend, you've got to risk having her hate you in order to give her the advice she needs to hear.
Tell her exactly how you feel. Get it all out. And if she refuses to listen or she just blows you off, talk to her parents about what's been going on. The alternative is sitting back and watching while she endangers herself, and that's just not an option.
She'll probably hate you...at least for a little while. But eventually she'll see that you were trying to help. If she doesn't realize that, then her friendship probably wasn't worth preserving anyway.
Note: this was a real question that a real person emailed to me! (It's been edited a little for the purposes of this article.) If you want to ask me and the rest of the Teen Advice community your own question, come visit the Teen Advice forum.