Don't say shit until you have concrete evidence to present.

A really good friend of mine's wife was fucking the best man from their wedding for a while. This was his so-called best friend from 2nd grade. She tried fucking me a few times and through all of this we found out she really got around during this marriage. I knew from day 1 she was a dirty whore, but what could I do? He wanted to marry her. ..So I knew about it for over a year, but I couldn't say anything because he was pussy whipped and I knew if I tried to say anything she would deny it and throw me under the bus which would kill our friendship. I knew in due time he'd find out. One day that "friend" left his phone sitting on the table, so I started going thru his pics and there were naked pics of his wife in it that she sent him so I showed him. Their divorce just became final a few weeks ago.

When my buddy asked me why I never told him and I said I didn't want to ruin our friendship and I knew in time he'd find out or I'd figure out a way to tell him.