Maybe she wants it for herself, maybe she wants something in the living room that the family can play together instead of little Timmy locking himself in his room and cursing out other xbox live members, or maybe she grew up playing NES and likes their franchises. Whatever the reason it's her money and she should spend it however she wants.
I got $5 a day for lunch at school.... nothing in the summer. If I wanted something I started raking leaves, mowing lawns, or shoveling drives until I had enough money to buy it. I'm really glad she didn't give in and encourage that kids behavior. There is way to much of that crap going on nowadays. She is spending the money and wants a Wii, if he wants something else it's his own responsible to get it for himself. Until then he can just buck up and enjoy the fact that his mom bought any gaming system at all.
Smash Bros: 2363-5325-6342 
