Well, I've only had 1 spinal surgery. I had 70% of my spine fused together T4-L3 to be specific.

Now to the question on hand. Your mom might want to look at a hybrid. I didn't know how my back would handle a bike, and I wanted to start riding again. So I bought a Gary Fisher Zebrano. It is an upright bike about the size of a road bike with straight handle bars. It has shock absorbers in the front forks and the seat post like a MTN bike. It also had wide tires, i think they were 700 x 42. It was a very smooth comfortable ride even on the bumps.

I sold it last year to upgrade to my current bike because I found my back was a lot stronger than I thought. But the Fisher bike was a good place to start for someone with a back issue.