My first thought was to condemn your SIL for getting rid of his things, but as you say, and I recall from dealing with my own M&D, it's very difficult and different when you're actually dealing with it. Still ... it's happened and the question is how to deal with that now.
He's already having some help, but I'm wondering whether the non-driving is really getting to him. A scooter-thing sounds like a good idea. I also wonder if there's some sort of volunteer driver scheme in his area. There is round here and it's great for a spot of shopping, getting to a club, or appointments, etc.
Have you talked to your SIL about what she's done and why? Or is the relationship a bit too delicate for that? I do understand if so. Family dynamics and 'expectations' are very difficult to deal with.
Have any of you been in touch with Age UK? They also may have a volunteer befriender/visitor scheme in that area?
I'm also wondering how much physical help your FIL needs. Is his 'feeling' that he's getting old and feeling the effects, or is he really in need of more help?
Sorry, this is a bit waffly as I'm typing as thoughts occur to me.
Good luck with it all. Your FIL is very lucky to have you caring about his mental well-being, which is what it's all about really.
Best wishes. x
A new definition of what constitutes British

That isn't going to get things off to a good start. 