I seen a friend's spouse out on a date with someone else, should I tell my friend, or mind my own business and leave it alone?
No stay out of it... I was in the situation myself I thought I was doing the right thing telling my friend about it. instead I got caught in the middle of drama. Because of it I lost a good friend..
Tell your friend
Yes. Tell him. He will be more devastated if he finds out on his own.
Tell your friend. One way is tell your friend and make it easier by saying I wasn't sure if it was just a friendly date or emotional.
Make sure that your friend's spouse IS actually involved in an extramarital affair before you do anything! If you are sure... well... Use your own judgement... you know your friend better than anyone on here.
Tell. You'd want them to tell you right?