Hi Kathy
There are a couple of things you can do for your student before beginning a yoga program to help with her knees: 1) Ask her what her doctor/physio are recommending? It would be useful to know just how severe her condition is and, 2) Read these two articles on
"Yin Yoga vs Restorative Yoga" and
"Yin Yoga for the Knees."
The challenge we all face when presented with an injury is knowing how much exercise to do: osteoarthritis usually develops due to too much stress in the joint, so the natural reaction is to not stress the joint at all. But too little stress will be just as problematic and could lead to osteoporosis. Your student will need to do some exercise for her knees, but the best thing you can do is help her become more aware of her own body, to learn how much is too much and what is just right. The exercises in the second article may help her a lot, but she will need to start very slowly and gage the effects of the exercise over the next day or two. Only when she knows she is okay would she even consider increasing the amount of time in the postures or going deeper.
Mobilization and stress are the keys to health and healing, but you will have work with her to gauge how far and how fast to go. Inner awareness will be key.
Good luck!
Bernie