What should you do if you know for a fact your dad is cheating on your mom and your mom doesn't know and your dad doesn't know you know?

Im sorry, but regardless of the attached, you can NOT control who your mom or your dad dates. BUT you can help it. This is not my situation, but my mom and dad got a friend divorce and i know what im talking about. So, if i were you, i would do this: Make sure you know for positive that he is cheating, because if it was all a misunderstanding, the cosequences could be severe. Get proof, then go to your mom and tell her everything. Tell her to kindly talk to your dad, and you may or may not want het to bring you into the situation. But, there is a possibility that you might have to. For example, if you dad denies cheating, you will have to tell him whatever proof you had. If he continues to come up with excuses, just fight with your proof. He may get you in trouble, and you can accept that. But he will eventually confess. But,take the matter the right way, and face the truth. All over all, you can not stop your dad from doing whar he is doing. Best wishes. Im sorry about the situation your in, but regardless, you can make it better.