You are very kind to help your friend pre-plan; I'm sure it will help alleviate some of the stress for both of you.
First of all - don't plan on the courtesy wheelchairs that are (supposed to be) in the parkings lots being at all available; we have many reports here that indicate that frequently none are to be found... or if they are, they are broken and either unstable or nearly so. Plan on whatever works best absent a parking lot courtesy wheelchair, and *if* you happen to find a good one, then it's just extra bonus!
Secondly, I too share
@SirDuff concern about your friends' use of the ECV. Can I recommend that you, her son, and she all head to a local Wal-Mart, Target, or grocery store, and have her drive one of the ECV shopping carts through the store
at a busy time to see how she does? This should give you all a much more realistic idea of her abilities; although the
ECVs you will rent at WDW are somewhat more nimble, they are still larger than a standard external (non-Disney) rental, and move at closer to the same speed. If she has trouble negotiating the store, I can assure you that she will not be able to handle WDW as the crowds will be even more intense at times, and even more closely packed. I'm a very experienced ECV driver; I own my own units; and I use them daily... but there are times at WDW when the crowds are still overwhelming to me.
Remember also that the Disney Parks
ECV rentals often "sell out" fairly early in the day; hence the popularity of off-site rental vendors. You can "hop" a Disney Parks ECV rental from Park to Park each day, but again there is no guarantee that an ECV will be available at the next Park you want to go to.
Next, I have some concern for her nighttime vision - remember that not all areas of the parks are brightly lit, and the headlights on most ECVs do little to compensate. If her night vision is at all impaired, she may not be at all safe to drive the ECV at night. There are many places around WDW where the lights are intentionally dimmed to bring your attention to a feature, attraction, or view - and if she cannot see at night well, she will not be able to drive.
Last, if she has trouble walking, she may require the assistance of a mobility device just to get around at the Resort, let alone in the parking lots. There are options - renting a lightweight, folding wheelchair, or even a Rollator might be something to consider so that you have it with you outside the Parks. If you decide to go to Disney Springs, remember that it is now nearly twice the size it was just four years or so ago - so you will want/need to have a device available there too. You didn't say if you were staying onsite or not, but regardless, it might be beneficial to consider other options for her. A lightweight chair that her son could push (at 13, he may be strong enough - and 13 year olds bounce back A LOT faster than grown ups do! LOL) might be much safer, and less stressful for everyone.
I hope that you all have the best trip possible - don't rent an ECV just because it's "free" (and I'm curious how she got a free Parks rental? Never heard of a voucher for that!) and then have a stressful time trying to help her navigate the Parks. I hope you find a solution that will be safe and comfortable for everyone who is a part of your trip!