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

I would suggest to take pic or video as proofs as it cant be "misunderstanding" when the couple is holding hands, the guy's hand over the lady's shoulder or any other intimate actions involved. If you were to confront the guy without any proof, you are just giving him chances to lie or to badmouth u in front of ur bff. Best way i think is to take pic, sound the guy out and also hint ur fren about it. If things doesnt get better, please inform ur bff. U wouldnt want another person to tell her and when she told u, she found out tt u actually knew it long ago. Put urself in her shoes, she will feel more hurt if u knew it and didnt tell her. (Based on own experience)