I have a former ex that always appeared when I started a new relationship causing them to fall apart. We would resume dating only to part within months. Years later she showed up when I was to be married, being older and wiser I wished her a good life and goodbye.

I agree with much of the authors response...she is dating your close friend...did either ask how you feel about it? I would write them both off and move on with your life. They both must be aware that it will hurt you but obviously do not care. If he is a close friend he knows how you feel about her. She was probably staying close to you after the breakup to access him.

If cutting her off cold turkey brings her back to your door...do not answer it.