I don't think it is right for the son to tell the mother about the father's infidel ways. this issue is quite weighty and needs careful disclosure. no one wants to believe that their partner is a cheat and revelation may elicit a lot of emotional responses. the best thing for the child to do would be to confront the dad and let him know that he cheated on the mum by having an outside affair. depending on how the dad may react, the son may choose to disclose to the mother. though the pieces of evidence remain in the child's mind, it would also be important for him to disclose to a third party who may then inform the mother. this may be a family counselor.