I highly recommend renting an ECV. It does depend upon how fast the rest of the party is used to moving, but one time my husband wore a pedometer and found that he had walked 11 MILES in one day. It can be several miles if you are covering just one park for the day. That, along with standing in long lines can be tiresome.
I just took 2 very Pooh sized friends to
Disneyland. He is 425 lbs and she is 277 lbs. They successfully rode Grizzly River Run, Soarin Over California, Pirates of the Carribean, Big Thunder Mountain Railroad, Splash Mountain (he was in the back seat which seemed to have more room), Star Tours and Indiana Jones. Now, were they both comfortable in every ride? NO. But they rode anyway, squished into some of the seatbelts. My friends were just about in pain after riding Indiana Jones actually, so I would avoid it. We waited to ride Matterhorn only to get a closer view of the new seating and realize there would just be no leg room for my friends at all, so we got out of line.
If you want to avoid any last minute issues with seat belts not fitting after waiting for a ride, I would suggest: Pirates and Big Thunder for sure. Pirates there are no constraints. Big Thunder just a lap bar. It's a Small World is another great and often overlooked ride.
She can enjoy Submarine Voyage (Finding Nemo) in the alternative viewing area for those in ECVS and you can either take the ECV in or leave it just outside and sit on the bench they have in there.
There are so many more things to Disney than just the rides. If she has an ECV, even the seating in inside theaters would not be a problem because she could sit int he ECV if she would like. I am talking about DCA Aladdin, DCA Bugs Life, DCA Muppet Show, and Disneyland Captain EO.
There are all the parades and street shows, the food, atmosphere. The street vehicles on Main Street, Great Moments w/Mr.Lincoln, shopping and so much more. Hope this helps.