I dont know, but, make sure whatever she gets will meet her needs.
How far will she travel?
Will she be using it on the lawn/grass? what about dirt or gravel roads?
My MIL has severe arthritis and both of her knees need replacement, but surgery scares her. She moved to FL to get away from winters, she can still walk, but, she cant go very far, a scooter lets her get out and live her life.
She likes to ride around on her lawn, so she got one with some pretty chunky tires/bigger wheels. Not sure of brand/make/model or any of that.