Welcome to planDisney Carey! I appreciate you bringing us your question. I am so sorry to hear that your fellow Guest has hurt his ankle - such bad luck and timing. Let's try and ensure he (and your whole party) enjoy the best time together at the Most Magical Place on Earth.
First, I can confirm that
knee scooters are permitted at Walt Disney World. They should have at least 3 wheels and be able to maintain stability and balance when stopped. Devices should be single rider and not exceed 36" (92 cm) in width and 52" (132 cm) in length. I believe that you can use them in the queues for attractions, you can always confirm with the Cast Member at any particular attraction. Since he may not always need the knee scooter, you can leave it at the boarding area, or on my recent visit, I saw many parked next to strollers while the owner was likely to have been enjoying a ride. It's best that your fellow Guest checks with his doctor about what he should and should not do with his injury.
Please remember that Walt Disney World Resort is vast, and moving around while injured may be painful. A potential alternative to the knee scooter may be using an electric scooter (ECV) to avoid issues that could arise from that much use of a knee scooter. You can
rent an ECV right at the theme parks.
Carey, I hope this information helps fill you all with a little pixie-dusted hope that your visit to Walt Disney World Resort will be magical! Please let us know if you have any other questions, Louise.