Should I tell my best  friend's boyfriend what she does behind his back?
Why would you tell on your best friend?
I think he deserves to know the truth. He's also one of my best friends.
I think it's okay to tell a close friend if their partner is cheating on them. But make sure you're really doing it to help him, not out of spite.
Fine. I'm going to tell him, maybe I saw something else on my way home.
Be sure to confirm what you saw is correct. Otherwise you might cause issues where there were none in the first place. If something is happening then they deserve to know.