I would wait for us to have the conversation about her again. Then I'll advice him and hint him to keep a close look on her, even if he can start tracking her movements. I would have gathered my own evidence too. If he eventually find out, then I'll show him what I saw too...
You see your friend’s wife in a lounge, all lovey-dovey with another man.
But your friend always complains she’s cold, distant, and rejects him at home.
Now you know something he doesn’t, but it’s not your marriage.
Do you tell him… or stay out of it?