There are a lot of back issues which people can have. I have problems with my back when standing or walking, but sitting, I am fine. I love to drive.
If your friend can comfortably sit, I think she could enjoy driving. The bumps don't bother me at all. Riding in a car is worse for me than driving.
Morgans make good driving horses for sure. She'd be advised to get a trainer to train one or both of them. Then she could take lessons and enjoy the thrill of driving.