I would never try to hide anything from Mrs. Alura, simply because I know I've never been able to. She's the same way with me. Once she tried to keep a surprise birthday party from me. Someone slipped and said, "...when Ken comes over tonight..." Surprised, I ask, "Is Ken coming over tonight? I thought we were going out to dinner for my birthday." The look of guilt that crossed her face as she said, "No," made it clear that Ken WAS coming over and she knew it. The party would not be a surprise.
She refers to that as the only time in our marriage she tried to lie to me.
I'd suggest telling the other man that you have no choice other than to tell your husband and then do it. You and your husband will then have to decide if y'all are willing to "live a lie" with these folks, unless the other guy is willing to come clean with his wife.
Maybe the four of you could agree for your husband and his wife to have sex alone to "even the score." I don't know if that would work to keep the friendship together, but you might consider it.
Mrs. Alura just read this and would like to add, "If he's not willing to tell his wife, you should stop playing with them. I would not suggest trying to "live the lie." It will not work."
Mr. Alura