When my XH found out that a previous partner was cheating on him, the first thing that he did, was to go and tell her parents and the OM's girlfriend. That was his first thought and action.
When I discovered last year that he was texting OW who is married to his best mate, I kept quiet initially as I was hoping to get XH to come back and stupidly didnt want to upset him by telling his friend..... then after I decided to divorce XH, I didnt tell the friend as I wanted to secure the financial settlement first.
By the time I did communicate with OW's H, they had already minimized the contact, and he did not believe me. He said that she had a lot of male friends, that XH was his oldest friend of 30 years and would not do that to him, that XH was not her type, blah blah blah. (XH and his mate could be twins, by the way...).
Her H completely excused away the thousands of texts and emails and flirting. he would not read anything into any of it. There is none so blind as those who will not see.......
OW turned on me completely and winds XH up over various things, contact, maintenance etc. I was accused of trying to ruin their marriage
, she is still with her H.
If you are going to tell, do it because you feel that her H needs to know, in order to make his own decisions, not for revenge. and do it sooner rather than later.