I believe the Wii U will have plenty of more accessible games in the coming years, especially focusing around the use of the tablet controller. I do believe it will lack many of the casual motion control games due to 1) The novelty of the Wii's motion controller back when it released caused a plethora of these titles and 2) Wii U's main controller doesn't focus on motions/swinging/etc.
I think it comes down to the types of casual games she'd be interested in: motion or tablet.
Also, do remember that the Wii U is completely backwards compatible, so while it may cost him more upfront, down the road he could end up saving money if his daughter ever wants one. Plus she'll have more game options with a console that can play 2 consoles' worth of games! =)
