As an avid user, I can truly say it works.
The Wii Fit is an excellent way to get both adults and kids active in a playful, effective way. You can make it as serious or fun as you want, and I think your nephew is going to focus on the 'playful' part of it.

The Wii Fit has a large range of 'standard' fitness and yoga exercises, as well as a lot of games involving balance. The games are very tempting, involving soccer, hula-hoop, walking on water in a bubble, boxing, and several others. you need the balance board to participate, and -while I don't play the games that often myself- these are very, very addictive.

This is something Nintendo has really covered perfectly: getting people involved, participating, and KEEP them that way.

However, as with everything, what you get from the Wii Fit depends on what you put in. If you stop using it after the first weeks, there is nothing the Wii fit can do for you (or your nephew) to slim down. If your nephew uses the Wii fit for at least 3 times a week (and the game itself keeps track of that, and will comment if you stay away for more than a few days!), you're definitely going to see an improvement.

Oh, if your nephew doesn't have the Wii Fit yet, you might tell his parents to get the fit Plus] instead! It's a newer, better version of the same program!

There haven't been any longitudinal, double-blind studies done... but...

Plenty of people have let go of weight using Wii Fit. I use it three times a week for my upper-body regimen (my treadmill is my lower-body).

There are some really good, standard exercises (push-ups, crunches, etc.)... And there are some fun games that exercise the body.

It works!

I got the Wii Fit a few months ago. But one must remember that it is still a game and not a training regiment. It is exercise but running on a treadmill and going on an elliptical, strength training with weights and machines is better. However using the Wii Fit in one's free time can help you get stronger and improve your posture and strengthen your muscles, which are all benefits to using the Fit. Of course, when those are accomplished, one who is playing Wii Fit will start to lose weight as well. which makes working out easier. I think the yoga poses, hula hoop, and strength training are the best. The running is good too, but you can always go on a treadmill for that. I have the same goal you do. People have lost weight using the Wii Fit. Just exercise for real and then use this during your free time. Combine those two and one will lose weight easily.

Mainly, your nephew should use it inconjunction with a healthy diet and 30 minutes jogging each day. What is most important for him at his age is to eat less snacks and foods with a lot of fat. I think his mom should should start to train him to eat healthier, because there are long term benefits associated with that, Wii Fit is only temporary.

Here is what Forbes had to say about losing weight.

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One of the things that's made Wii Fit so popular is the excitement Nintendo has stirred up with people who don't normally play. For Crecente, that excitement--and the let down that often comes with it--hit home. "My mom and step dad both tried Wii Fit before it came out," he says, "and like every other middle-aged American, they loved it." In fact, they loved it so much they went out and bought a Wii of their own. Months later, though, when Crecente stopped by for a visit, he didn't need to ask if they'd set foot on their balance board. Their Wii had never even been set up. So much for physical fitness.

Don't blame Nintendo for people's sloth, observers say. The company has marketed its new cash cow brilliantly, and it's not responsible for whether consumers play the game or not. Nintendo declined to comment for this article, but Wii Fit creator Shigeru Miyamoto has previously gone on record and said that the game is less about people losing weight and more about broadening the videogame market. Still, it seems a little disingenuous for Nintendo to heavily market a fitness tool that's sitting in more than a million American living rooms collecting dust.

That's not to say Wii Fit doesn't work--if you play it. Brian Ashcraft, another Kotaku editor, tried the game out faithfully for a month when it was first released in order to review it. The results: He enjoyed the yoga, and started to feel more in shape. But the novelty wore off, and Ashcraft admits he hasn't picked up the game in a long, long time.

Not everyone, however, is prepared to give up on Wii Fit. Instead of relying on anecdotal evidence, Scott Owens, a professor of exercise science at the University of Mississippi, has started a six-month study to uncover whether placing Wii Fit in a home will actually improve a family's physical fitness. By donating the game to local participants for three months at a time, then taking it away for another three months, Owens will be able to observe how the game impacts cardiovascular fitness, flexibility and balance. How often families use Wii Fit will be up to them.

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