I too had a "friend" like this. I stuck by her for years, acting as her patsy & getting her out of trouble with others (despite her regular attacks on me) because I felt sorry for her, understood where her problems came from, and felt she needed me.
It took years to realise that despite knowing that she couldn't help herself, those attacks on me still diminished me inside. It still took a couple of traumatic events - one being the unexpected death of one of her daughters (I think she resented the fact that my only child, one year younger, is very much alive. She'd always resented anything I had that she lacked), before I finally got free. And I still feel guilty for "abandoning" her after such an event, even though she instigated it.