No problem! Wife and I did Grand a couple of Christmases ago. Make sure you book a handicap
cabin, so that your friends' WC can easily navigate in and out of cabin and more so in and out of the bathroom. The HP staterooms are right next to a bunch of elevators and there is an excellent access throughout the ship. Princess does a good job taking you off the ship, but be carefull what land excursions you book. Assuming you are taking a Western Caribbean cruise, none of the Carib Islands are "HP-friendly". What I do, once my wife is helped down to pier, I take a taxi and we do the Island "things" on our own, than I know what is accessible and what is not.
In brief, remember: the newer the ship, the better accommodations for the handicap person.