If the Rebel has less than 18 megapixels, a body upgrade is in order.
If sports isn't a priority and you want to stick with a crop body, the 70D paired with a Canon 17-55 f2.8 would be a huge improvement and an ideal system for general use.
However, if full-frame is in play, consider the 6D with a 24-70 f2.8L II or even the older 24-105 f2.8L.
As previously implied, there's a risk here. These suggestions aren't cheap and we are making assumptions about your wife's interest in photography. I can only draw from my experience. A few years ago, I upgraded from an 8 mg XT to a 60D. For a short zoom, I chose the 17-55 f2.8. I was blown away with the IQ improvement and the low light performance of this body and this lens.
Eventually, with my sports photography, I pushed the limits of crop bodies and ventured into FF with a 5D3 and a 24-105 f4L. I was blown away again with IQ improvements and better low light performance.
Full frame is expensive (and heavy). It typically leads one to buying more 'L' lenses. It may then lead to buying better post processing software and better computers. But, once the money is spent, for the serious photographer, it can be very rewarding.
General advice is not to upgrade until the limits of your gear are holding you back. You need to decide whether the limits of your wife's Rebel, or of crop bodies in general, are constraining her photography.