Hi Claire - first, it is great that your student is taking matters into her own hands and working to regain her her health and functionality. It is nice of you to assist her, but first and foremost let’s recognize that a yoga teacher is not a doctor nor a therapist. Your student seems to feel abandoned by her primary health care team, but this doesn’t mean she has to be alone. She can and should seek out a doctor who will continue to work with her, not ignore her. You state that she is “in rehab” so it is not like she has no support. This leads me to ask, “what does her physical therapist recommend?”
She could certainly adopt and adapt a Yin Yoga practice to keep the rest of her joints working well: no need for standing exercises in Yin Yoga, so no need to stress her feet while in the yin postures. Almost all the regular Yin Yoga postures should work, with perhaps some padding for the foot when necessary. It should be okay to target the hips and spine, and upper body.
However, if you are thinking of adding some postures to directly target her injured feet, well … that is a different topic: I would be reluctant to offer any advice about that. It would depend upon what her therapists are recommending. Unless and until they give the okay, avoid postures that directly stress the feet: avoid Toe Squat, Ankle stretch and perhaps postures that work the ankles should be avoided for now, like Overstepping Dragon or regular Squats.
You asked about pain: again, if we are targeting areas not including the feet, regular advice still applies. No pain! (See my recent article Good pain/bad pain?
. However, if again she is targeting her feet, pain will be natural since this area is damaged. It is up to her and her therapists to distinguish between the pain she needs to experience as she stresses and heals her feet, and the pain that is making things worse. As a yoga teacher, I would default to the old adage: “no pain, no pain!” But again, that applies to the rest of her body. When she works her feet, she will have to learn the difference between healing pain and hurting pain. Not easy, but she sounds determined to heal, and that attitude is one that you can help her foster.