As was mentioned by SueM earlier, Randy's will sometimes rent to a teenager on a case-by-case basis. My guess is that a 17 y/o who has full mental capacity and fairly good upper body control, will most likely pass their requirements, but there's no way to know for sure. The other thing to keep in mind with an ECV vs. a wheelchair is that an ECV will take more energy to use than being pushed in a wheelchair. If she gets fatigued easily in ways that impair her concentration, a wheelchair may be safer.
As others have said, there's no GAC that will help with distances walked or time spent standing. They may have an accommodation for some of the queues that are in the sun, but that isn't very many and most of the exposure will be between attractions so the suggestions of finding the right sort of protective clothing are going to be most helpful. Also, keeping hydration up is really important (and that's something that everyone should be doing so should be able to be managed without her feeling like she sticks out). If she has any problems with overheating, those mini-fans that do a misting spray may also help. You haven't mentioned what time of year you're going, but if it's not booked yet, it's worth looking at going in a cooler part of the year (my most comfortable WDW trip temperature-wise was end of January, but even October/November is better than any time in the summer and not too crowded which makes the fatigue easier to deal with in my experience).
I've been going to WDW using a wheelchair (either my own manual wheelchair or a power chair) since I was 22, 6 years ago. I'm used to using a manual wheelchair self-propelling and I need to be pushed at WDW so if that's part of her problem with using a wheelchair, you can assure her she will be far from the only one being pushed rather than pushing themselves. I've seen people of all ages using wheelchairs and
ECVs at WDW and 99% of the time, no one has ever particularly looked twice that I've seen. I look relatively non-disabled when using a wheelchair, along with being able to walk/stand very short distances, and I've never experienced negative comments, even when getting up out of my wheelchair. I also tend to look younger than my age (part of the medical condition I have) and was carded at WDW as recently as when I was 26 or 27, so I would think that I'd get comments if anyone was going to. It is also possible that I've been having so much fun with my friends when I've been there that I haven't noticed the comments or looks, but as far as I'm concerned it's a "tree falls in the forest" situation. If I don't hear the comments or see the looks, then they haven't happened as far as I'm concerned.
I had one trip to WDW prior to using a wheelchair (by about 8 years) and before my family and I were really willing to admit that I might have mobility issues. I can say honestly that I was in much more pain and it did impact my enjoyment of that trip. Even once I started having more obvious problems I forced myself through some family trips (not to WDW) without using a wheelchair and it was miserable; I would spend about half the trip sleeping in the car or hotel or in too much pain to do anything. While I can understand why when you're at a point of recovery that it would be frustrating to feel like you're taking a step back, your friend will find herself much happier and have a better trip if she can spend her energy having fun rather than just getting places. Also, it's worth letting her know that people use wheelchairs and ECVs at WDW for all sorts of reasons; I would guess that close to half don't ever use a wheelchair or ECV in their daily lives, but
either have a temporary injury or a medical condition that makes the 6+ miles a day of walking at WDW too much, even if they're fine around home.
It doesn't matter whether they ask or not. If you read the contracts for every rental company I know of, other than Randy's, they state that the ECV cannot be used by anyone under the age of 18. When you agree to that contract, you agree that the ECV will only be used by those above 18, which is the part they care about. Most companies ask at least for an approximate weight of the user because of weight limits on the various models, but I'm sure there are some that don't. I would expect that the minimum age requirement has to do with the insurance that the rental company has. If someone under 18 is using an ECV that has been rented by a company that states that the user must be over 18 and the user gets injured or the ECV gets damaged then I would assume that it would void any liability insurance and the renter would be responsible for all costs. If it is an ECV rented in one of the WDW parks and a CM notices someone under 18 using it, they can and will take away the ECV; I'm not sure if they would have to refund the deposit in that case. Do people lie and/or otherwise try to get around the minimum age limit without getting caught? Probably. But it's certainly not something we can advocate doing on this board.