Hi Chiffon. I agree with Joanne. Having trained a basketball player with JRA, I know that it is vitally important that you familiarize yourself with the disease and ALL of its implications on exercise programming. FIRST of course, have the client obtain FULL medical clearance to exercise, and require that any exercise limitations and contraindications to be spelled-out by the medical professional.
If there is any doubt about your ability to safely work with a client with this, or any other health condition, you SHOULD NOT work with them. Their health and safety is the primary concern.
I hope that this helps.