I mean, does the dad have an obligation to know with who his son was in love years ago? If i would hate someone, it would be the girl. She reappears after some time just to join his father's harem out of the blue? And if she was gonna do that, why the fuck does she kiss him the day before?
Dunno if after reading the story there would be some kind of reason why she did all this, but at this point if i were the MC i wouldnt touch her even with a stick.