when I first started in to photography when I was a kid, my father handed me a pentax k1000 and a ricoh 50mm f1.7 lens. He wouldn't let me use any other gear until I could take good photos with what I had. When the day finally arrived that I got a canon eos with a 28-200 zoom lens, the quality of my pictures went in the toilet, and took a long time to get better, because I became too preoccupied with all of the choices I had. Once I realized this, i went back to primes, and I rarely get very far away from 43mm. That 50mm f1.8 canon lens is amazing for the price, in fact, it is the lens i have on my eos today. If I were in the position today that my father was when I was a kid, i would do it no differently. I would hand a new photographer a fully manual film camera with a 35 to 50 mm prime lens, and make sure they have the basics down before they move on to anything else. Even though everyone loves new gear (I know I do), it is far more important to be familiar with the gear you have, and to concentrate more on the composition than the tools.
my advice to the op is that when your wife is ready for a new lens, don't get her a zoom, get her another nice prime, perhaps an 85mm portrait lens. Zooms tend to make people (in my experience, myself included) lazy.