I never wear a belt anymore, due to peer pressure from the Oly lifting coach. The work on improving my form - as opposed to relying on the belt to protect my lower back - has been positive. I don't think I would have done it otherwise, and I'm glad I did just to improve my technique on the squat and DL. I had become used to wearing a lot of of protective gear in strongwoman training. When I did the Colorado St Fair Strongman contest, I did wear a belt for the overhead press medley, as it was on an unstable surface.
I have used a 4 inch suede 10 mm belt for 4 years, and it works great for pretty much everything. I have an Inzer Lever belt with the tapered front, and I like it as well. The only thing I don't like about it is having to use a wrench to readjust it. My experience has been with 13 mm belts, double prong, single prong, lever belts, etc. I found that a good single prong 4 inch 10 mm belt will serve her well, and last for years. Hope I can help in some way in your search for a belt for your wife.