I HIGHLY doubt I could be with him after he cheated. However, if we were still together (and it would have to be for a VALID reason) then the question of "ALLOWING" him to still be friends with his side piece should not even be a question. If it is then there would be no relatonship.
His inner voice (conscience) should be his only guide on whether it is advisable to be friends with someone he cheated with. If I have to point this out to him then we are done. It would be pointless.
I do believe in the saying "men are like dogs, if they are not trained right they shyte all over your carpet." However, it is not my job to train him. If I have to tell a grown man who he can and cannot be friends with, we have a big problem.