I assume they're both young.
Sounds like the sort of thing that fits a younger persons thinking.
Child rearing is hard work !
Trust me I did it with three under 6 !
If you have a parent that is immature and can't differentiate between what needs to be done for the benefit of your children and wanting your own carefree life, then let them go.
Last thing you need is to have to care for an older child who will always have the power to walk anytime life gets tough or they don't get their own way.
But don't think that just because they walk today that's the end of it. Because it won't be !
If they have told their own parents about it, you can be sure he'll be back contesting that decision at some point.
The power of the Grandparents to suddenly make their wayward kids 'take responsibility' is bigger than you might think !
And the worst kind of parent is the 'I wanna be responsible today but maybe not tomorrow'. Because children need reliable parents 24/7. Even if they hate each other !
I guess what I'm saying is that whatever decisions you make today be aware that life moves pretty quick and people can change if they're under stress or if they realise they have family support they didn't realize they could count on before.
Relationships can be stressful. Even for older people who have children.
So try not to be too judgemental.
After all you're both trying to find a way to make this work.
As long as you keep the idea of what's best for the children at the centre of everything you do you won't go far wrong.