Speaking from experience, as the one who was cheated on.
Have proof to show your friend. While you want to trust your friends and believe that they are telling the truth. You want to believe your spouse 100 times more.
This is what I fear that she will believe the spouse over me. I will lose a friend or if I dont, I will have to worry about the husband or the mistress trying to ruin my life. It sucks you walk into the right place at the wrong time and see what you didnt want to see and now you are caught up in the middle of something you didnt want to be involved with.