aussie,
It is a strange thing, but sometimes our friends choose to self destruct. I view this as a person to person option, as I really don't know your friend. So here are what I view as your two options:
1) Hound this girl, get her to seek help. Get others involved and see if you can talk to her before she does something stupid.
2) Sit back and watch. I know that sounds bad, but sometimes people have to hit rock bottom and make mistakes before they will learn from them. Keep yourself at a distant but try and be there when she needs someone to pick her back up.
Just my personal opinion.
- Jonathan