One of my friends is currently in a relationship with an ex of mine. Me and her were together for a good 3 yrs and we had broken up on good terms and are still close friends to this day. He told me 8 months later that he and her were talking seriously about getting into a relationship and asked me for "permission." While he didn't really need it, I did appreciate that he was up-front about it and was honest about his intentions. Both of them even invited me to their apartment for a few threesomes. They look great together and their relationship is 5 yrs strong. My point is while your timing of hooking up with her may have been off, your honesty with him will go a long way towards your friendship with him.
The situation might be put to rest. But regardless I think it's a thread that can continue on about the general idea of how you handle dating the ex's of your friends/relatives. I think it's ok in general, but obviously you want to be aware of specifics. Obviously dating an ex that burned your friend badly or broke up badly with them and then putting both him/her in a social situation with your friend/relative is a bad idea. But in the above guy's situation that wasn't an issue because the breakup was cordial and mutually agreed upon.
IMINMYUNDERWEAR, I caught you in your first gaffe; you should not have dignified ENDNOW's comment. You must be slipping. ENDNOW, iminmyunderwear is demonstrating wit, which unfortunately is in short supply. While I can present a non-sequitur, I in no way can match the man's gift of verbal touch. Both of you - have a drink on me. OP, follow your conscience - it is serving you well on this one. cheers, mate - JKHolman