WIBTAH if I never tell my wife that I slept with someone else while she had cancer (even though she gave me permission)?

Many years ago, when my (53M) wife (48F) was in her early 20's she got cancer and it wasn't looking good.

In the midst of a bunch of serious conversations we were having at the time, the topic of intimacy came up. I tried to reassure her that, that was the farthest thing from my mind and that my main priority was her well-being. Despite that, she said she felt guilty and insisted that if I found someone else, she was giving me permission to go be with that person.

I didn't plan for it to happen but as my wife got sicker, one of my wife's friends - Mona (46F) - was a really big help at this time. Over the course of a few weeks, we bonded over taking care of my wife and one thing led to another. We were extremely discreet and thought it would be prudent not to let my wife know (not because of guilt, but because of the weak state she was in and our desire to see her rest and recover). The relationship continued a few more months after the doctor said the cancer was in remission until my wife and I were able to become intimate again.

I immediately and regretfully broke things off with Mona who understood and we parted on good terms. She graduated, left the state, met a great guy, stayed in touch, and even invited us to the wedding. Unfortunately, her husband passed away last year and she eventually moved back and now lives about half an hour from us. We visit her. She visits us. Everything was great.

Last night, we were all hanging out and my wife started telling Mona about how she had given me permission to sleep with someone else and I had never done it. Mona laughed it off but I could she felt guilty. When my wife went to the bathroom, I asked her, "Do you think we should tell her?"

Mona said (from what I can remember), "I'm trying to put myself in her shoes but I'm so caught off guard right now that she didn't know. She said she gave you permission, but how serious was she when said that? Maybe she meant it at the time, but look at how happy she was saying that you didn't take her up on it. And take into consideration that it's been more than two decades at this point with no problems. Should we maybe just let this lie in the past? I'm leaving it up to you. If you won't tell, I won't tell."

I've been thinking about since then and I don't think I should tell.