Your "mate" is a selfish f@#!, and I think he is only after your "ex" for sex, nothing more, he used you and your exs "troubles" to his advantage to get to her.
I hate people like this, I honestly think you should do something, but like I said, be careful, the guy don't deserve to die or anything, but he definately needs to learn a lesson.
Honestly, I know you won't believe it right now but, you WILL move on and someday you won't even remember this girl.
I was with my gf for 3 years, I thought she was the love of my life, and i'd be with her till I die, and she got pregnant to my "best mate", not just sex, not just a fling, she got pregnant...
He got dealt with though, eventually.
Be strong man, and don't give her an inch.