Telling One’s Father That He Committed a Transgression

Shalom u’vrachah 

A son may not say to his father, “You transgressed the words of the Torah.” 

It is permissible to tell, in the father’s presence, stories about people who hurt others and were punished, or about people who asked forgiveness and were saved. 

If the father asks on his own, “Is there something I need to correct?” it is permissible to answer him in a respectful manner: “Perhaps Mr. So-and-so was once hurt by you.” [But they should not say, “You hurt So-and-so,” or “You behaved improperly toward him.”] 

Certainly, the sons should try to help as much as they can to get him out of his difficulty, through assistance, encouragement, and the like. But they should not “help” him by conducting his “soul-searching” for him...

Shulchan Aruch, Yoreh De’ah, siman 240, se’if 11
If he saw his father transgressing the words of the Torah, he should not say to him: “You transgressed the words of the Torah.” Rather, he should say to him: “Father, it is written in the Torah such-and-such,” as though he is asking him, and not as one warning him; and he will understand on his own and will not be embarrassed.