Looking at the video she was being a bytch and the guy was being petty.
He had the computer, he just rather put all of that together on Xmas day, not a big deal, plenty of kids have to assemble their gift on Christmas if it's coming straight out the package anyways, it's a normal thing
She starts insulting him, made Christmas about how she wanted it to look and used her son as a shield, knowing damn well that kid wouldn't care about putting to together on Christmas day, that shyt is damn near a rite of passage
He called her bluff

extremely petty cuz the kid is gonna be sad.
In my experience when women get disrespectful over small potatoes like that she was already looking for a reason, threw shade at his older kid for having the new computer. Pure hating
He was probably done with her too seeing how fast he just said fukk that kid
But this is what happens when you try to leverage kids feelings against another adult, he was probably equally disgusted that she weaponized her kids Christmas trying to be a control freak.
I bet money if she asked the kid he would've been fine putting it together in the morning.
As someone's who mother has also tried to make up POV's to attack the dad

she's a manipulator, need to fix that now while the kid is still too young to tell her about herself. Women like that have a hard time keeping men around em for long.