I kinda agree about the overall wisdom about H not telling the OMW. What would it accomplish but inflicting pain in a situation in an uncontrolled fashion? H would be doing OM's job and if OM were a smart man, he would start being a 10x better husband for a few years before telling his wife, and then after that, he'd have to crank it to 100x for the rest of his life. OMW is in her own little world thinking she only has little worries. Why destroy that?
More importantly, I think H should have a man-to-man chat with OM and tell him it's OM's job to tell his wife. Regardless of the response...and I mean REGARDLESS, your H should stare him down, call him a gutless wonder and walk away. That's my view of where it should start and end. If H needs to say anything, then it's to the man and then walk away. The power in controlled anger and the ability to walk away peacefully would speak more for H than for OM.
Besides, if the A is over and there hasn't been any contact, then there's no secrecy left to expose. I would only tell the OMW if the A between the OM and W was still going on...just to stop it. Selfish, but it's very present, in the moment. But this isn't your situation now.
I think if the conversation ever came up, I would tell H to put the reponsibility back on OM. He hurt his wife and he needs to tell her. If H tells her, then H is hurting the OMW. Not a good thing.
H42 W36 M9 yrs D8 D5 d-day: 21/11/07 S and moved out: 22/2/08 Still S: 22/11/10