@buggi My sympathies and I do understand that aspect but reading it all together, Her mum hasn't jumped at a mobility scooter as a straight swap for a vehicle, its been a slow realisation or acceptance of the fact, she has a spouse who doesn't sound like he'll give her an easy ride on it (no pun intended), was once an active participant in a sport, and has stated a specific intention for its use. Agreed she may become more dependent on it or opt to use it for a journey or two or more more, but she may not also.
My point is that we are individuals in our own heads & we have different levers working on us externally too, but you seem very clear & dogmatic in how this total stranger to us both will conform to a particular norm of your choosing.
I've gone on enough about my 2011 hit and run mashing my knee and at 47 awaiting a replacement in a few years. I get the debility thing & lifestyle upheaval painfully clearly and have a nice comfy car. I have had to enforce a mental discipline on myself not to let it slip and I go for the easy option (car, short Brompton hop and tram, bus from outside my front door etc) when I must not because I can.