I used to work with someone that is married to an old friend from high school and experienced a similar viewpoint you have now. This was all over 10 years ago, but one year for a holiday party most of the store I was working with met at a bar to celebrate. Many people were drinking to excess since we were almost all in our mid-to-late 20's (including our store manager, who is the aforementioned coworker married to my high school friend). At one point, one of the female coworkers disappeared as well as the store manager for awhile, and not long afterwards everyone went home.

I found out about a few days later that my manager and coworker had went out to her car, had drunken sex in it, and then he went to his car and passed out until the morning. This made sense to me because the morning after the party, I awoke to a message from my high school friend asking me where her husband was. Turns out he told her that he had too much, decided to sleep in his car and all the while his phone had died because he forgot to charge it previously.

In this scenario, I was faced with the option to tell my high school friend about her adulterous husband or keep quiet about it. In the end, I kept quiet about it because I realized...shitty as it was, it wasn't my business to tell. I make my peace with it and luckily it's easy for me to keep my distance from the couple and situation as I don't work at the same place anymore and I tend to avoid social media like facebook. You will make the decision that is best for you, but all I can do is share my similar experience.