Not experienced in this particular scenario but I live by a policy that dictates if any one of my friends puts his penis before our friendship I'm happy to cut them loose.
Cutting people loose in general is a bit of a high school term, and I wouldn't completely blank them outright, but I'd make it known through my mannerisms and general demeanour that I didn't give much of a fuck anymore.
I despise it when people lose all sense of reality in search of a sexual thrill. There's probably countless numbers of women that your mate could've banged that night, but he didn't, he banged your ex. That says everything (to me) that I need to know.
Happened to a very close friend of mine, I had him this same advice, cut ties, they're obviously a complete fuckwit. He didn't. Said cunt came and stayed with him, uninvited, ended up trashing his house after an MD binge and leaving unnannounced one morning with a pair of his shoes.
Complete cock.