Sell your Wii fit to me then for a bargainous price. I'll offer more than what a paperweight is worth!
I'd say it is a good present, sure, just make sure they're into some of the games. There are lots of singing/dancing games that would be good, and don't forget the classics like mario cart.
The machine can also use iplayer properly now, and browse photos via SD card, so it's not a one trick pony, though a lot of the titles are total gash with not many good ones on the horizon.
If she gets bored of it you could always sell it on to someone who doesn't have one.