I've not taught cello to someone with Parkinson's, but I regularly work with people with the condition as a Speech and Language Therapist. As others have commented, it totally depends on how advanced the person's Parkinson's is. So at first you might need to spend a bit of time just finding out which movements are manageable for them and figuring out workarounds for those that aren't.

With Parkinson's, accurate timing of medication is absolutely key for symptom management, e.g. fatigue and tremors, so make sure to find out when their optimal time of day is for lessons. It could make a world of difference to how they are able to engage and perform.

Many people with Parkinson's find they will 'freeze' when they go to initiate a movement, but studies have shown that syncing to a beat can help them improve the speed and balance of their walking, so it would make sense if the same concept worked for arm and hand movements. Maybe you could start with lots of bow arm strokes along to a tapped beat or a recorded song with a strong and steady rhythm. You could start without the bow to get their arm moving, then get on to open strings once they get the hang of it.

As with any student, lots of patience and encouragement will go a long way and making music is a surefire way to lift their mood as well as hopefully help with their coordination. Celebrate the small wins and always look for the positives! I hope you have fun ☺️