I have a question... does your wife *want* to lose weight? Or is it an issue of your wife wanting to be/feel accepted as she is, however that might be?
There are a lot of fat people out there who do not *want* to lose weight. It's far "healthier" to learn how to accept oneself than to attempt to change into something that you're not.
Maybe you and your wife could look into something like fat acceptance together?
There is a lot of very encouraging stuff out there. I know that if my sweetheart decided to become my weight loss coach it would really turn me off. I've come to a point in my life where folks who don't fully accept me and love me as I am don't get to run around with me in any capacity. That includes "concern trolling" in regards to my size. When people give me unsolicited weight loss tips I generally will try and speak to them about why it is that they really are barking up the wrong tree with me... and if they just doggedly ignore what I'm saying, because, after all, "obesity is a disease" and they've been propped up enough in their crappy mentality by the mass media, well. I just disagree, move on, and they kinda go in that "ones that mind" category, if you know what I'm saying.
Another good book you can share with her, if you like, is this one:
And here's a link to the wiki on "Health at Every Size":
Maybe it would help if your wife saw that you do indeed love her as she is, and would encourage her to be accepting of herself, and not just encouraging her to change herself to make everyone else happy? Just a thought, from a fat girl who's been through a LOT of fat girl stuff. :-)