The best way to figure this out is to talk to them. I think the worst thing to do is to just see what happens. If you do that you may not find the answer. So . . . Talk to them. If they get defensive, then something is probably there. If she is really your friend, and he really loves you, then they wouldn't you
If you think your husband and a friend is messing around, how do address the situation without making a fool of yourself and straining a friendship?
If you can afford it have a detective follow them. If you are right it won't take over a week and then you will know the truth.