You want to make sure it won't get caught up in the wheels, so it should cover her lap but not necessarily hang down much more.
It depends on if it is a wheelchair she moves on her own, or is pushed by someone else. One has the big wheels and one has smaller ones.
You might want to take a yard or two of fabric and put it over her while she's sitting in the chair so you can see where the fabric falls, and then you'll know what measurements would be right - for HER. Her weight, her size, her height.
I hope she has a smooth rehab period. That's a tough situation. My sympathies.