Hi Steve. I highly recommend having the client obtain her physician’s approval to exercise and work with you, and then to try to work with and through her physician and other healthcare providers that she may be seeing (e.g. physical therapists etc) to help your client in a holistic way. Backs and knees are two areas of the human body that seem to suffer injury the most, so taking the time and care to gather as much information as you need from your clients healthcare providers before proceeding would be very helpful to the long-term success of your client.
I hope that this helps.