Most guys I know are insecure, and have a problem with their friends dating their ex's. Personally I think its dumb.. if shes your ex and you dont give a fuck about her.. you are going to get mad at who she fucks? That just shows her you still care about her, and then she has you by the balls.

I personally want all my friends to fuck my ex, to confirm what she really is. If shes not my girlfriend I dont give a fuck who she has sex with. I seem to be the minority however. You say you arent friends with him anymore, make sure you mean that because Im willing to bet he would hate you for fucking her.

If you know you wont be friends with him anymore then I guess go for it. Good luck man