I have never been in your shoes,God Forbid! but i can only imagine what you've been through. Hurting the dude wont help but i assume being his friend it would be easy for you to find out what his weaknesses are. You might even already know. Example may be: his unauthorized Employment.

Love hurts and that dude should not have banged your girl knowing the fact that she is your girlfriend. So, go ahead and report his unauthorized employment (if you know of) to the authority. This would teach the dude a big lesson and you can be happy.
Forget the Bi(ch. She does not deserve you.