About a year ago I met a girl that was in a similar situation. She chose to stay with him. I have backed off of her completely for the whole time, and just watched her ship sinking.
This was a very cute girl - but very messed up. I hadn't seen her in awhile and was amazed at how a cute chick could look so aged and worn in so short a time. She looks like absolute hell. Maybe it's the drugs, the stress he puts her under - but I always remember one thing - it was her with the problem, her who couldn't let this son of a beech go - and it's her that's paying the price in her life. She could've gone with me - the interest was there - and she would've been better off - she knows it, her friends know it - and every now and then I catch her looking at me and I can read her like a book - I can see it in her eyes - wondering what could've been. Her loss, completely. Not mine, because frankly, the chick is so unstable she would've driven me mad. I look at her now, and my interest-level in her is pratically zero on the romantic scale - what a complete turn-off she is to me now.
I've learned alot - especially not to get hung up on a girl because when you take a step back - you can see very clearly. It was for the best that I never had anything to do with the freak.
As Befuddled so poignantly pointed out - people get EXACTLY what they deserve.