Kathryn didn't help as much as you think...
Yeah, she was caught in the middle, trying to keep the peace.
But she knows Pat sets Luke off...
She was fine absorbing all of Pat's flirtatious attention...
And she was consistently more gentle, sympathetic, acknowledging toward Pat, while firm, correcting, even dismissive toward Luke.
She kept defending Pat in her conversation with Luke.
She asked Luke, why do you think he's spending time with us?
No sweetie, he's spending time with you, trying to get into your panties, while taking every opportunity to tell you how Luke isn't good enough...and you're not shutting it down, not defending Luke, at all to Pat. Just tacitly saying, Luke can't see you getting handsy with me... Then Pat, you're perfect, that BOY doesn't know how lucky he is...

Come on Kathryn, get a fucking clue...
And then, Kat asking Luke to leave, she didn't go after him, didn't ask him to wait in the car, or outside, nothing...
She didn't say no, I don't want to stay without you, when Luke said stay.
She let Luke leave, so she could stay with Pat, alone...
Sorry, Luke deserved better from Kathryn in that scene...
Kathryn chose Pat over Luke.
Luke should dump her and move on...
In case it's not clear...
I cannot stand Pat, at all... So Kathryn can go fuck herself for wanting to stay with that shitbag...
If Pat wanted to actually do something nice for his son and Kathryn, pay for Luke's dancing lessons instead of dropping $400 for tickets so you can date Kathryn while your son watches... Absolute piece of shit...