There are a few considerations here. If you will have a car, you can consider the better modern hotels that are outside town - the Unique Colony and the Sheraton are both quite nice and well-equipped with spas, pools, etc. Without a car, these places aren't worth considering.
I've stayed at the Posada Plaza Mayor, which is dead-center in the historic part of town and reasonably comfortable. It's in a 200+year-old building where the rooms are arranged around a garden with a little fountain. Because of this, the rooms tend to be darkish but still atmospheric, with original stone walls and high ceilings.
There are some more luxurious in-town inns with pools and such that are a few-to-several blocks outside the prime historic district. If your mom is up to walking sometimes difficult cobblestone streets, there are a number of options you can find on tripadvisor or another travel site.
Finally, the Radisson is modern, well-equipped, and in-town, about 5 blocks from the historic center, overlooking the river near the marina. I've never stayed there, so I can't comment on the condition of the rooms.