I am assuming she had both wrists medically fused, and the bones did not fuse on their own.  She needs to keep the joints in the hands healthy and increase circulation throughout the hands. I would focus more on that than on strengthening, as it sounds like her hands are getting enough of a workout. Gentle movements linked to breath – start with hands in a loose fist, on inhale (one hand at a time!) open one finger, one knuckle at a time, on exhale close one finger, one knuckle at a time. Do 6-10 repetitions on each side (she can alternate hands if she likes). Also, opening the hands on inhale as wide as is comfortable, sustaining the stretch for a moment or two, and gently relaxing the hands as she exhales.

The first exercise of linking breath to slow movement can be done very slowly (have her do it throughout the day as well) and is a nice option when your class is doing wrist exercises. The other thing she could work on is cactus arms, drawing the shoulder blades together strongly on inhale, then bringing hands and elbows together in front of her on exhale…..just to keep her rhomboids strong for better thoracic spine and shoulder girdle alignment. I would also think stretches to keep her chest open, particularly pec minor, would be helpful to avoid compression on the brachial nerve plexus as it goes through the chest to innervate the arms.