I would be in this position if my wife died before me. I think that Motability give you a bit of a grace period of a month or two, which would be long enough for me to buy a used car. Personally, I wouldn’t expect them to do anything else. The scheme is for disabled people and the car has been VAT exempted as a result. As I’m thankfully not disabled myself I would fully expect to have to fund my own car.
Not that I would personally want to buy an ex Motability car, but I think in these situations they should offer the car to the bereaved partner at the current trade price (not the inflated prices that they used to ask at lease end). That’s all they’d get for it at auction, so it wouldn’t result in the scheme funding non-disabled customers. Maybe they do this anyway? Thankfully I’ve not had to ask!