One thing worth pointing out if you write to Motability is, your friend is actually your PA rather than just your friend. The thing with ringing, is the customer service reps are imo hopeless and the computer will say no!
At least by writing, you can take time to express your needs in more detail, such as your “PA” is a life saver to you and helps you all day doing “x” but will have to collect her children from school before coming back to assist you in the evening – On another day after assisting you, your “PA” will go to town to get shopping for both herself and pick up what ever you may need, such as your prescription and drop it off before going home. You can do this and lay it out in better detail without getting stumped or caught off guard if you were to do it on the phone.
Your PA living 3 miles away, won’t pose a problem, however, they may decide to fit a tracker as part of the condititions and monitor use of car as @Rene says, which means they will monitor reasonable use, to the benefit of you, so there as got to be activity of the car going to and from your address.
It’s certainly worth the effort to put it in writing, as 99% of the time it’s to your benefit, even if it’s just picking up and bringing back a pack of mints gor other 1% of the time.