Well knowing how custody works in Japan will make the mother seem less like an asshole. One parent almost always get sole custody of the child, usally the mom. There really isn't any split custody where one parent has rights to see the kid. The other parent is usually never seen again, mostly because the have no rights to see them. That's way most devorices happen after the kid went though high school.
Doesn't change the fact that she didn't even remember how many years she didn't see him for. Like the jump from 6 years to 3 was too big, then to not pay full attention to him at the parent teacher meeting and just stay on her phone.
But interesting to know how their divorces work in Japan
How trash mum did it all. Imo his mum was worse then the dad