I doubt that this other guy is the only issue in their marriage. Nothing happens in a vacuum.
I'm betting her husband is emotionally distant somehow. That's usually the case for situations like this. And what's the deal with the big age gap for your kids, OP? Are they both yours & his, or from a previous relationship?
At first, I read the husband as an ass wanting to have relationships outside the marriage, but that's not the case. He actually sounds like a reasonable guy based on the info so far.
Second thoughts can generally be amended with judicious action; injudicious actions can seldom be recovered with second thoughts.
--Cyteen by C.J.Cherryh