OP can you clarify something, please? When I read your earlier, early comments you mentioned having to pretend and that your husband and the woman's had mutual friends. Does that mean that you are having to see them socially and pretend nothing is wrong? Because in that case there is no way in hell I'd fake it for a couple of cuntbags like that. I'd either tell her boyfriend or (better) dump my loser of a husband and find one better!
If you are having to see and pretend and fake to friends then I can see why it would be exhausting. If they aren't your friends though, fuck them, don't bother to see them, If they are your friends as well then I would be honest with them and if they tell him, that's up to them, but I'd definitely be honest with them.
I have told someone her DP (my close friend) was cheating on her. He's a serial cheat, much as I love him to bits he should never be in a relationship. He's never not cheated and, in this case, he was on the way to breaking her heart because she was seriously falling for him. She was naive and sweet and a really nice girl, I felt I owed it to her to tell her what he was really like. She burst into tears but she was so grateful, he'd been convincing her nothing was going on but she'd suspected. Having that confirmation relieved her and gave her the push to dump him before he did her. My friend pouted for a while over it, but got over it. Now we're all very upfront and honest about how much of a womaniser and cheat he is when he brings them out. If they still want to try and 'change him' then more fool them. He gets embarrassed but he's knows it's the truth and I think he's relieved not to hide it.
Someone told my friend her DH was cheating on her, worst possible timing, just after she'd had their firstborn. The text didn't help at such an emotional time but she had been suspicious and asked him and he'd lied to her about it all (obviously) even before the text. When he started acting that way again and proper gaslighting her, making her feel paranoid and stupid, the first thing she did was text the number and ask if her husband was cheating again- just on the off chance they knew again. Turned out it was the OW's sister who texted unknown to the OW (who that time hadn't known she was OW). She answered honestly and said they'd started things back up again and she'd had her suspicions that he was lying about being separated again but her sister was too weak to listen. Since she'd got no response last time and my friend had stayed married to him, she'd seen little point in bothering again. She figured my friend knew and turned and blind eye and her sister would get a very harsh clue when he dumped her for the next one. My friend and her actually talk a lot on PM facebook now, she's helped my friend get through her divorce as well. Very weird situation, made for heart flipping moments when my friend showed me the messages.
I know someone who cheated, with a good friend of mine and she was horrified and so upset when I had to tell her he had a fiancé. I must admit I didn't tell his fiancé though she's such a lovely person and part of me really wanted to, just because he did her wrong. He really saw an honest side to me when it came to the cheating, I told him her was a wanker for doing that to my friend, and to his DF, I really was furious with him for close to a year. He was one of DHs friends first and they moved in the same group, DH didn't want anyone to know at the time and stated he knew how sorry his friend was and that it was a one off according to the guy. He was furious with him and said it's no excuse but he couldn't bring himself to do anything to bring the news to light and cause issues. I suspect now that it's not a one off, I know he cheated on his last two girlfriends. I refused to socialise with them one on one though for that year, because I refused to lie or pretend. This year, it turned out that all his other friends knew which prompted him to very belatedly admit to her what he'd done, which was a weight off my mind because it meant when I saw her I felt I was being sincere and not holding back the truth and pretending everything was fine. He is still with her but whether he'll do it again, or has done, who knows. Three times now, three different women...sounds like he'll always chose to be that way.