I wish I had some advice for you, kayaker, but I think you’re in danger of losing a friend any way this comes out. It happened to my wife and I.
We had a large group of good friends - we’d vacation together, and saw each other fairly often, despite being pretty far-flung across the city. Over the years, we all began to get hints that one couple was having problems. The husband - I’ll call him Reginald “Pongo” Twistleton-Twistleton* - would be gone at odd times, such as Thanksgiving and Christmas, and come back with … strained… excuses (“My company is Korean, and they don’t understand that Americans get Christmas off.”) One time I encountered him walking with a young woman, not his wife or daughter, in a park some ten miles from his home, at dinner time when I knew his wife, whom I’ll call Dahlia Travers*, would be home fixing dinner and taking care of the kids. At first, I just kept running and didn’t acknowledge him, but then decided to turn around to check; and he had disappeared.
The group’s misgivings were only that, until the family of the young lady - Bobbie Wickham* - managed to contact some of us, my wife included, on Facebook. That’s when we discovered that Pongo had been essentially leading a double life for years, spending weekends and holidays with Bobbie and her family. She had no idea Pongo was married; he had told her that he was divorced and that Dahlia was mentally unstable. Her brothers were trying to confirm this when they reached out to some of us on Facebook.
So the feces hit the aerator. Bobbie broke off the relationship, and Pongo came clean to Dahlia. And she apparently forgave him - they’re still together.
Pongo and Dahlia have three wonderful kids, and Dahlia is a devout conservative Catholic and deeply private; I obviously don’t know the inner dynamics of their marriage, but she still professes to be devoted to him and her family. However, she’s ruthlessly cut off about half of our group; one, Cyril “Barmy” Fotheringhay-Phipps*, because he was Pongo’s close friend and one of the only people Pongo had told about his affair. The rest of us, on various pretexts; she cut me off ostensibly for a Facebook post when I savaged the Catholic Church over the clergy sex scandal. She’s cut her ties to most of the group, and I haven’t had contact with her for about three years. I miss her, too; she and I were pretty good friends, and running buddies who had done races together.
Again, I can’t say what’s going on in her mind, but it certainly looks like she’s furious at her husband, but pretending otherwise for the sake of her kids, and redirecting her anger to the rest of us.
*P.G. Wodehouse characters whose personalities bear no resemblance whatsoever to the real people I’m talking about.
tl;dr version - Adultery damages all relationships, not just the marriage. You’re in a bad situation, and, unfortunately, you may lose a friend no matter what you do. I hope it doesn’t happen to you.