My best friend and Ex having strong feelings for eachother what should I do?

You know what? you should try talking some sense into your friend. Just straight up tell her that he's not a good guy. He cheats and sleeps around and isn't a trustworthy guy. Tell her you don't want her to be with him, not because you're jealous, but because you don't want her to get hurt like you did. If she still doesn't get it, then there is nothing you can do. I don't think your friend is very considerate about how you feel either. She knows what he did to you, and not only is she friends with him, but they're practically dating? Honey, move on with your life. Let them do their thing and move on with your life. There are plenty of fish in the sea (cliche i know, but true). Good luck!