this question is really not specific enough. Parkinson’s comes with different challenges and degrees of progression. As such, there is not one answer. If you can qualify your question, I may be able to provide more information. I have trained one client with PD literally from diagnosis to his death last year and thus have witnessed all the stages of PD in his case.
What is generally true is that you MUST have a physician’s approval. The client may have seen a physical therapist who you should contact for information. Parkinson’s is extremely medication-dependent, and the timing of the medication may influence when your client can optimally workout.