It is absolutely none of my business,it is my father's cup of tea to stir.
Though, that act would definetly severe my relationship with my friend but I have no axe to grind with my father. As a matter of fact, I won't mention it to my mother, so far he also keeps it from her.His philandering is his choice. God help him.
But if he disrespects my mother (probably by waving the illicit affair before her, trying her patience, questioning her relevance as I have seen some men doing such), I won't say a word to him, will only placate my mother but will pray God to punish him. PERIOD.