She sounds very troubled, but not like a sociopath to me. Has she actually been diagnosed with this or is she feeling bad about actions she has taken in the past?

Maybe the best thing you can do to help her is to model a healthy relationship. Have clear boundaries with her. Don't let her take advantage of you. Call her out on bad behavior. Treat her like you would treat a good friend, not someone you feel pity for or someone who has had a tough life.

I would also prepare myself for the possibility of losing her friendship without taking it personally. Sometimes people who are that bad off need a lot of time and practice to get better.