what will you do if you saw your best friend's husband dating another girl?

I can only say what I would do if I were you. I would approach my friend's husband and ask him about what I've saw. This is to give him an opportunity to explain in case what I saw was not what it seemed to be. If he is indeed having an affair, I would ask him to come clean to my friend. Otherwise, I would tell my friend. I cannot stand cheaters and I think my friend definitely deserves the truth. How my friend would deal with it will be up to her. No matter what, I would first check with her husband and give him a chance to tell her himself.