Wow – you have a very special case! In general with any surgery, the only one who can really say what is possible, and what to avoid, is the surgeon. Only the surgeon will know what he or she did during the operation: how much damaged they had to do to the ligaments in order to do the carpentry work, what was preserved and what was severed. So the most important thing your student needs to do is to talk to the surgeon after the surgery and ask what happened, and what rehab should be done, and when. Undoubtedly, your student will be assigned to a physiotherapist who in consultation with the surgeon will develop a rehab plan. Your student should tell the physiotherapist that he is planning to do yoga as well, and then take advice as to what movements should be avoided.
I know that after arthroscopic surgery on my knees, which was nowhere near as significant as what your student is about to undergo, I did find Virasana and Vajrasana very helpful, as well as balancing on one leg postures. You may want to view my article on yin yoga for the knees
and how see I rehabilitated them. For balancing work, given your student is 72, standing him by a wall would be a good idea: there he can support himself with one hand against the wall.
Having said all this, I have never worked with a student who has had both knees replaced. I would be greatly interested to hear if any other Yin Yoga teachers out there have had this experience and to know what they found helpful.