How can I convince someone to leave an unhealthy relationship?

My best friend has been dating a guy for about three years now. They are the most dysfunctional couple you’ll ever meet. Her boyfriend has a lot of psychological issues, and it places strain on the relationship. They’ve broken up more times than I can remember, and every time she insists that she’s “done.” My best friend is very smart and logical, and she recognizes that the relationship is problematic. She is aware that she should leave, but she just can’t follow through. I can think of a lot of reasons why she stays; the problem is how can I finally help her shake free?