What would you do?
This is purely hypothetical, but a scenario I have heard played out half a dozen times at least.
You are friends with a married couple. One day you see the husband out to lunch with another woman. You sit back and observe to see if your suspicions are correct, that he is doing something he shouldn't be. At the end of the meal you watch as they leave, he has his hand on her back and she looks at him fondly.
You never see them kiss or hold hands, but your gut tells you something isn't kosher. Do you mind your own business or say something to the wife?