So they have an arrangement in place, it works.
He gives her the dates in advance (and closer to the time too)
Then... he comes in and suddenly wants more dates when he knows he’s off work?
It’s not making sense? If I had kids I wouldn’t want mine being carted off because my ex suddenly decides he has extra time either to be honest!!
There are many things that his ex would have to consider (routine, budgeting, mum and son bond etc) before she could just let her son off and go to his dads (or on a holiday!) it’s not so simple as you think.
And consider yourself lucky- if he went through court isn’t it usually every other weekend plus old night a week? So he’s having more than most!
Does he pay child maintenance???
It’s also not your fight is it!? Let him deal with the consequences of his choices (choice not to go to court, choice not to “stand up to her”)
Because these are his choices, you aren’t his mother!
Her social life, and her childcare/ support from family or parents (when she is effectively a single mother the rest of the time) is none of your business!!