Or at the very least, the ex wife deliberately forced him into doing the dumping. I don't see a man, with the character the OP has described, who won't leave his unhappy marriage with his childhood sweetheart for the willing woman crawling all over him and begging him to be with her, just up and leaving to be alone and single only around year later, entirely of his own accord.
Perhaps the ex wife, who had been tormented by suspecting an affair, forced his hand so that he 'technically' ended it. Whatever he told the 'mutual' friend group (who all but one abandoned the ex wife and sided with the cheating husband) doesn't really count – it seems like they'd all happily keep secrets or tell lies for the husband.
It's interesting but really very pathetic and slimy to see someone trying to justify carrying on in failing marriages for around a year, lying, deceiving, and cheating (and if they were still having sex with their husband/wife, it really is a form of rape, ethically if not legally, as there is NO WAY their husband/wife would've consented if they'd known), instead of doing at the start what they did in the end, anyway!
Just say it's over at the beginning, and begin the process of separation. Marriages end without affairs all the time. THEN you can go shag each other like rabbits. There's absolutely no excuse for cheating, especially cheating that carried on for around (over?) a year, all the while their marriages were floundering and dying. It shows a total lack of any moral integrity, sense of responsibility, or compassion for others – its an incredibly self-centred, emotionally immature thing to do, and given neither of them regret it, I imagine they (or the OP's husband at least) will find it easy to justify to themselves the next time it happens.