Unfortunately, I think pushing your wife towards dieting may just make the problem worse.

When I first started putting on weight my ex informed me I was getting fat, which he probably meant as a helpful statement (ex or not, he was a decent person) but I took it the wrong way, so my attitude was "I'll be as fat as I want to be!" (Have I mentioned I'm an idiot sometimes? )

It took until I hit my personal breaking point to *want* to change. People can be pushed into going through the motions, but they're not going to change unless they have some want in them.

Hopefully your success will inspire her to want to change as well, and when she does you'll be there to help her on her way.