If you saw best friend's spouse out cheating do you tell the best friend?
The answer is unequivocably yes.
I just so happen to have experience in this field, unfortunately.
I had a friend who had a cheating spouse. It so happened that Mrs. Toxick knew the cheating spouse from way back (small world), and ran into her from time to time. We ran into her running around on her husband. (She didn't see us, however).
At about the same time that we caught her, someone else also caught her, and told him everything. He didn't believe this person (who, in all fairness, is not the most reliable person in the world, and is a known drama starter), and stopped talking to them.
When this happened, I decided to hold my peace, because I didn't want to lose him as a friend.
When it all came to a head, he ended up finding out the truth, and later he asked me if I knew about her. I told him that I did. He asked me why I never told him. I explained that I would have told him, but he already knew the truth from someone else, and he didn't believe them.
It was at this point he said something that still rings in my ears: "Yeah, but I would have believed
you."
And to this day, I kick myself when I reflect on that.
If it's your best friend, tell them.