Gee... this is such a tough one Lucy...
I went off the rails myself at about age 15, and left home, got into drugs and all sorts of other terrible things.. and I never really came out of it until I met DP, and then fully got over everything when I fell PG.
If you want I can give her many gory stories of my own.
During my wild times I was attending a 'Flexi' highschool, basically for young homeless and/or troubled teens. So I have seen a LOT of this.
The sad thing is... even after having lived through it myself, I really dont know what any good suggestions would be. As everybody is different. I know I have changed soooooo much, but I know of friends I used to have back then who are now worse than ever.

Hopefully it is just a stage she is going through. I remember when I was at mainstream highschool nearly everybody started to go feral once they hit year 9. But most of them do come out of it. I think an important thing is to keep her in school, and keep her living at home if possible. Cause once the stability of those things is gone, and she is out on her own in the big bad world as a teenager, it is dangerous, but highly exhilarating, and I doubt it is very appealing to want to go back to normality and 'straight' society for ages. Usually it takes many hard knocks and traumatic experiences for them to realise that its not as great as they thought. But by then it can be too late, as it has become a way of life.
Sorry, I am totally rambling here!!
I will stop now... sorry if I am not much help.
Feel free to email me if you want any real life stories to scare her with...

Best of luck to your friend.