First time I went to Disney I was recovering from a knee fracture and walking long distances was a real issue, so we arranged one of the wheelchairs and it did us proud.
Couple of things worth noting:
1) The Disney wheelchairs are not the most comforable in the world, and I found that repeated small adjustments were needed to stop aches in places I didn't know I had :lol:. That said, I imagine that it's be the case for all "generic" wheelchairs.
2) Some rides were just not accessable in a wheelchair (though I imagine the ones that weren't won't be of interest to your mom anyway).
As I remember they were pretty cheap to rent but you had to leave a deposit as well (which was refunded of course, in either regular US or Disney Dollars).