After my marriage finally crumbled to pieces because I just couldn't hold them all together anymore I went to a therapist for a bit. She told me one thing that you should also keep in mind here.
You can't fix someone else.
Believe me that I understand the desire to help and to try and the compassion that drives that desire, but in the end it's a futile effort. You can still care and you can be grounded and supportive, certainly. However, absolutely by no means should you get deeply involved. By the time that you realise that you are in fact trying to fix her, (most likely against her will, people with these sort of problems don't like to be fixed) it will be nearly too late. You'll find your entire life torn apart by someone else's problems. I'm speaking from experience here having spent 7 years of my life sinking deeper and deeper into a hole that a person was digging for herself, it's like trying to help a drowning person, there's almost more danger for you than to her. Almost 4 years later I'm still not shed of all the damage. Don't fall into that pattern man, people with that level of problems will self medicate in just about any way that they can find and there will be nothing you can do about it. She's going to have to come to a point where she wants to get the help for herself and to get fixed herself, no amount of you caring or pushing or pulling or anything else will make her better or make her want to be better. Like Vudak said, you can be her friend, be encouraging and be stable and that's the best thing you can do for her, but always keep in mind that you can't fix her problems for her and you can't make her want to fix them for herself, she won't fix them for you or anyone else, no matter how bad you might want her to.
It's one of the hardest things in the world watching someone destroy themselves bit by bit, but it's harder still to have them take you down with them.
You can't fix someone else.