Have you tried any work on a stationary bike? You could progressively add resistance on the bike until they are able to do more weight bearing exercise. You could also do some intervals on the bike to train different energy systems (15 seconds on, 15-30 off, repeat).
Other than that you should look into getting some r-Loop bands. You can have them work against the resistance of the elastic band while laying down on a table. I would just work on progressing them to be able to do weight bearing exercises.
My favorite exercises for those with knee pain: hip abduction and adduction movements (clam shells), step-ups to low box (focus on weight in heels), and static holds in squat stance or balance work on uneven surface.