Its none of your friends business if his ex wants to get into your pants. Its up to you as the friend to say no to his advances. The issue is between you and the guy, he tried it on with you and its up to you to tell him that you don't appreciate his advances because you have a bf and you aren't into him, and that its inappropriate because he is your friends ex. Thats all that needs to be said.
I wouldn't even bring it up at all. If that happened to me, I woulda just laughed and said "yeah right" and sipped my drink. End of. This guy can try it on with whoever he likes, he is single after all.