Tempting offer, I know she was abused by her ex, but all the same should I let her have a second chance?

Basically she can't resist her abusive boyfriend and you can't resist her emotional abuse of you. Don't get drawn into a relationship with her again. She is bad news and it is possible that her abusive boyfriend could someday become abusive toward you. Date other women; you will eventually find someone who is worthy of your love. Truly, don't waste anymore of your time with her because in the long run it just isn't going to work out and I think you know it.