I don't know what you should do but I can relate a similar story from my life.
We must have been somewhere around 20 years of age, and we lived in the same apartment complex of a couple who had attended HS with us and who got married because the girl has got pregnant.
"Beth, "the wife was a close friend of my wife, and had been a close friend of the girl I dated before marrying my wife. So I knew Beth for some time, and she was one of my least favorite people.
"Dan," the husband was new to our HS in our Senior year and became one of my friends. He was a complicated guy and someone who if you didn't like, you probably couldn't stand.
Dan was mad for Beth and delighted when they married. Beth was always looking for the romantic stranger to come nto her life and sweep her away (for a little while she thought this was Dan).
Dan beamed thought the wedding ceremony and Beth sobbed uncontrollably.
They had a son who was a cute as could be, and who seemed to keep them tied together for the first two years.
Shortly after her son's 2nd birthday, Beth started an affair with someone she had met in college.
I know she regets it to this day, but my wife told me about the affair after Beth confided in her.
That night we visited Dan and Beth in their apartment as we did on most weekends.
We smoked a couple of joints and drank some wine. I felt good, but not intoxicated.
At some point in the evening, the subject of extra-marital affairs came up. I honestly can't remember who brought it up and in what context, but eventually the subject turned to what you would do if you knew your friends's spouse was having an affair.
My wife and I talked about that evening quite bit thereafter but neither of us recall how the topic turned to the subject of this thread. We both thought it was entirely possible that Dan had steered it there, but couldn't say with certainty.
In any case, the conversation lasted about 45 minutes or so, during which I expressed the very emphatic postion that if I knew my friend's wife was cheating on him I would tell him, because I would want him to tell me if the roles were reversed.
We didn't leave for another couple of hours, but when we did, I thought my wife was going to kill me. It actually took me a little while to figure out what she was mad about. As unbelieveable as it may seem, I truly was not thinking about Beth's affair when we discussed the subject earlier. It really never crossed my mind.
Needless to say, Beth would not have bought that and what she heard was me threatening to spill the beans to Dan, if she did not. Soon after we left their apartment she confessed. My wife thought that was my intent as well, but I can only tell you it was not.
Dan and I never discussed the topic...not ever, and so I have no idea of what he thought I was up to or whether or not he appreciated it.
I can tell you though that the revelation that night not only led to the rapid decline and end of their marriage, but it has a terrible impact on him. Dan was always a very cocky guy and it was this aspect of his personality that a lot of people disliked in him. I always found that he could easily make fun of himself and so it didn't bother me.
However, when he found about Beth, the insecurity that apparently drove his bravado mushroomed and he became insufferable. He lost most of his friends and, eventually, I stopped seeing him.
Again, I was not concious of what I was apparently doing that night, and so I can't say I would change anything that happened, but based on the result I can say that in a similar situation, in which my sub-concious hadn't taken control of my brain, I would not have told the betrayed spouse what was going on.
I have no idea which is the best path to follow, as I'm sure if I had not forced Beth's hand, and she had kept up her affair I would have been livid every time I saw them and realized she was decieving him.
For whose sake do you tell the betrayed spouse?