Princess Cays is a tender port. That said, as long as the person can get on and off the tender, from there they are fine. There are concrete and wooden walk ways that will get you to all the facilities such as the store, bathrooms, local vendor booths , the bars and the food service area.
Just about everything else is beach and sand stuff which of course will not be terribly wheel chair friendly.
I have certainly seen wheel chairs accomadated on tenders in the past.
Might I suggest that someone Email Princess customer service and ask this question directly of them.