I don't agree with H should have a man-to-man chat with OM and tell him it's OM's job to tell his wife. It is not your husband's job to tell some other guy how to run his life.
Regardless of the response...and I mean REGARDLESS, your H should stare him down, call him a gutless wonder and walk away. Ok, I don't get this either. This is what I was talking about by "spreading around more pain." What does this accomplish? What is the goal here?
Will it make your husband feel better if he insults someone else? Is that what it is? If he hurts someone else, he'll feel better? I don't see it, I don't agree with it.
Besides, if the A is over and there hasn't been any contact, then there's no secrecy left to expose. Again, I completely disagree. The revelation of an affair, long over, can still be very painful, and often needlessly so.
This is worth a counseling session if he is willing.