Most rides don't have moving walkways. Of the ones that do, only Peter Pan and the Tomorrowland Transit Authority can't be stopped as Cheshire Figment mentioned.
Here is a list by park of the attractions with moving walkways:
Animal Kingdom
Kali River Rapids - the usual boarding area has a slowly turning walkway. There is an alternate boarding area for people with disabilities where a boat is actually 'trapped' and held still for boarding. After everyone is on and is settled, the CM moves the gate which was trapping the boat and the boat is free to go. It will return to the same place and be re-trapped for people to get out. Here is a picture of that boarding area. You can see the gate in the middle of the picture. When it is up, the boats just go around the area.
Epcot
Spaceship Earth - the usual boarding area has a slowly rotating round walkway. The boarding area for people with disabilities is at the exit. When you board there, CMs can slow or stop the moving walkway for you. Here is a picture of the exit.
The Seas with Nemo and Friends - everyone gets on at the same entrance. The CM at the entrance boarding area can slow or stop the moving walkway.
Disney's Hollywood Studio
There are none with moving walkways
Magic Kingdom
Haunted Mansion - has a moving walkway at the usual boarding area. Guest who need the walkway stopped would board and unload at the unload area.
Peter Pan - has a moving walkway and a very short distance from when you enter the walkway until you need to have boarded. The exit also has a very short space. If you want to ride it, I would suggest watching for a while to get the timing down and get an idea of whether or not it would be possible.
Pirates of the Caribbean comes to a complete stop for boarding and unloading, but at the exit, you are one floor below ground level. To get back up, there is a steep moving ramp about 15 feet ahead of you as you get off. To the left of the moving ramp, down a little hallway, there is an elevator. You can use that without talking to the CM or doing anything special.
Buzz Lightyear - has a moving walkway at the entrance and exit. To get it slowed or stopped, you will need to board at the unload area.
There area couple of rides without moving walkways where you need to go on quickly; they are mostly thrill ride/roller coasters, so I'm not sure she would be riding.
With a cane, she should be noticed by CMs and allowed to board at the accessible boaring area. But, just in case she is not, it is probably a good idea to get a Guest Assistance Card that allows her to use the wheelchair entrances. There is more information about Guest Assistance Cards in post #6 of the disABILITIES FAQs thread. There is a link in my signature to that thread.
