Not knowing the extent of her handicap I'm going to make a couple suggestions.
First, some of the Japanese ZZ machines can be retro fitted with the hand crank conversions as sold by Sew-Classics and others.
Second, If she has upper body movement you could rig a foot controller to the back of her chair so all she has to do is lean back a bit and push on the pedal. The electronic controllers would be ideal for this as they don't get hot.
Joe