Can you create a link to the type of wheelchair she uses? They are all a bit different.
Off the top of my head, I would think making something like a pillowcase with holes for the handles. The problem is with out the info on which model she has, I don't know what to tell you on how to attach the bottom. THe one my mom had has a gap at the bottom of the back (where it meets the seat) you could add ties to the cover for this type of chair, but I know others are closed at this spot.
I would definitely add batting, probably 2 layers of wool against her back. Wool breathes really well.