I definitely would NOT say a word. Been there, done that and it was the worst three years of my life. Yes, three years!! Now, if she had set me to task to find out if he was doing things behind her back, that might be a different story. But, if I just stumbled upon a situation and didn't know for a fact what was going on, I would stay out of it. I might get a little cheeky and go up to them and embarrass them and make them think I'll tell their wives if they don't.
The only people in charge of protecting or shattering the trust between the couple are the individuals in the relationship in question. It's not me. That, I know for sure.