Re: Absent Patellae
My good friend has a knee 'held together with bailing wire' in his own words. He has been a Sandan for a couple years now.
I think you need two things:
1. More information. Find out what the limitations are - I know the patella is protective, and gives greater 'angle' for leverage, but did they tie the ligaments together? What does this guy's knee look like from an ortho's perspective.
2. Let the student drive - being very body aware and feeling no pressure to 'show off'. Any class you walk away from is a good class.
Have fun. I've used aikido as impetus to heal some pretty nasty old injuries. Healing is stronger than fear.