As you ask us, Na who we go ask?
such questions are really hard to answer. had your friend told the man and he died just the way he'd did as his wife confessed,dont you think that will be more painful to your friend?
personally, I won't go around doing amebo and telling one spouse that the other spouse did this or that.
had I been your friend, I will have met the cheating wife and let her know I knew her games and advice her to stop, that way my conscience will be at peace; if she chooses, she can stop it continue.
but you will never see me going to tell her husband .

Wet in concern your friend? Is he the man watchman? if he tell the man, who go tell am wetin him own girl friend dey for him back

Some people prefer not to be told.

You should always tell, is the right thing to do.

What happens after tha is not up to you. They take the information and act the way they want. They may not even believe you.

But when you tell you have to realise that you can never be friends with those people again. While one party might thank you, the cheating party will see that you are the problem and blame you for the demise of the relationship insteadof looking at their own behavior.

It's called shooting the messenger.