I spent a little time poking around the web and found a bunch of information, pro and con about the book "The China Study". Interestingly, most of the "pro" information is anecdotal and non-professional. There is quite a bit of "con" from the medical community (here is an example:
China Study Book Review). I have not personally read the book.
Learning and trying new things is important. Thinking critically is important too. I think it is very important to teach children to think for themselves and question everything.
There is a lot of pseudo-science floating around these days and once it gets into the mainstream, it's hard to get out. You can visit snopes.com for hours of entertaining reading on all manner of fakery that suckered folks at one point in time (and some of them just keep coming around).
I would encourage your wife and daughter to do some additional research. Thinking and research is lot like medicine in general (medications, rehabilitation, surgical procedures, etc)....just because it's easy (easy to do, easy to understand) and makes you feel better (says what you want to hear or physically makes you feel better) doesn't make it good for you. Lots of times, medications, surgical procedures, rehab, etc....can be pretty uncomfortable and make you feel worse (at least at first). Exercise is a classic example of something that isn't all that much fun to do, but is generally accepted as a positive thing for your body and mind.
If it was all just about feeling good, mentally and physically, we'd all be sitting around watching Oprah, smoking cigars and drinking beer.