I agree that a medical release is important. I also like to accept the challenge because often times Trainers are the ones who can help with the condition.
Most physicians cure with pills and your experience as a professional will get them better results.
I had a client that had such a major knee surgery she was dragging her leg. I help her so much she stopped physical therapy! It was a challenge and I did work on a lot of modifications, but in the end she is walking straight and no longer dragging her foot.
Most likely, you will be the best medicine for your client.
Hope that helps!