You are friends with someone. You two have been close friends for many many years. You spend a great deal of time with each other.
Then one day you see your friend kissing someone who is not their spouse. You ask them about it and say something like,
"I trust you. So I am going to be honest with you but I don't want you to say anything until I figure out what I want to do. XXXX and I have been dating for a couple weeks now and I really like him/her. I know I shouldn't but I'm not sure about how much I like my spouse. Please don't tell anyone. I'm just trying this out. If he/she doesn't work out I will go back to my spouse."
What do you do? You know both this person and their spouse. You havn't seen any problems before this. They have 1 young girl.
What do you do?
IMO, it would depend on how deeply you are involved with their lives. If you're just their friend, it might not be worth getting involved. If you're involved deeply with either one of them, it might be prudent to get involved. If you do so, I would tell the "friend" what your plan is and give them a timeline in an effort to push the decision to tell (or not) on them. If they confess, you're off the hook. If they don't, then you might want to question their friendship with you (assuming they know you're doing it for the benefit and not to be a jerk).
But the ultimate question to ask yourself (if you haven't already) is: what business of mine is it anyway?
If you can't come up with an honest answer, you might want to avoid the situation altogether.