Hallways are wide enough as long as people don't leave items outside their rooms -- ECVs, strollers, food trays -- housekeeping carts are big but the stateroom hosts are nearby and will gladly move the cart if necessary. Since it sounds like she does walk, as PP suggests maybe she would be comfortable standing up and walking past with the chair folded. She may or may not need to fold the chair to get into the stateroom as the doors are narrow unless you have booked an HA stateroom. I'm assuming she doesn't not need the chair to get around the room; it can be folded and tucked out of the way somewhere.
Some doorways have a rather large lip, mainly doorways to the outside decks. If someone else is pushing this shouldn't be a problem but if she is trying to self-propel it might be harder. Also some of the doors are heavy and that's one of the complaints from people using mobility devices.
Try using the forward or aft elevators instead of midship. They tend to be slightly less crowded. And be willing to get on in the opposite direction if necessary -- for example, to go from the atrium to the pool deck get on an elevator going down and then ride back up.
Will she bring the walker as well? How does she plan to get around in the ports? Many are not as accessible as the US, and some excursions may or may not allow wheelchairs. You'll want to keep that in mind when planning your days.
Enjoy your cruise!
