That must have been painful for Peter to hear from his own daughter
Katie favours Mitch over Peter -- her own father (he's no deadbeat father like many) but it's also needed to take into account it's a mere child so she's only being honest (nothing wrong with that).
Peter should have never got himself involved with a nutcase like Lou. He's just too good for her and they don't appreciate him.
The heart wants what the heart wants.