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.
of course, you need medical clearance for this client and do an assessment as you would do for any other person.
With a history like this, it is rather certain that she has been in physical therapy for some time and should have PT instructions for exercises. With a bit of luck, you may even be able to talk to the physical therapist even though the surgery was a few years ago. You may also get some information from the doctor about restrictions, if there are any.
I usually start with the PT exercises and the results of my own assessment and slowly take it from there. Sometimes, vigorous cardiovascular exercise is not a good option, no matter how desirable to support weight loss. Losing weight, though, would certainly help, and 30 lbs. fortunately is not such a big number to make this a goal which can only be reached in years.
I wish you good luck.