After reading threads and posts here, I chose Walker Mobility this year, well on our Dec trip. Previously rented in the parks. I recommend Walker Mobility very highly and will definitely use them again next Dec! The cost was a little less than half of renting in the parks. Plus, can't take the disney scooters from one park to another or anywhere outside the park. (If you save the receipt, and present at the next park, there is no further charge than what you paid at the first park).
Also, it insures you will have the
scooter whenever you need it out of the parks. You can take it on the buses, boats or monorail and it's fairly easy to get the knack of driving onto these transportation. They are helpful in getting your scooter loaded and give helpful advise and can even park the scooter on the bus for you if needed, though I found it pretty easy. Since they are staying at boardwalk, it's probably faster just to drive the scooter over to Epcot so no waiting for a boat which can take some time. Not very far at all, and can also drive scooter over to DHS if they want, it's just a little farther the other direction.
She should bring the charger with her each day just in case, but I found that if I charged for 8 hrs overnight, the walker battery lasted the next entire day.
Practicing is a good idea, but it's fairly easy to get the hang of. Just gotta watch out for speed and for other guests stepping out in front of you.