The answer, to our delight, is "yes".
I had the opportunity to chat with a family member who spent a lot of yesterday with my injured mother in law.
When my mother in law had a dizzy spell and fell over the weekend, Bil was in the house. He heard her yelling (it did take him a while as his bedroom was two levels down from where she fell) but he then came and got the phone for her, and she called 911.
The family member also reported that Bil is helping her by fetching objects, and showing (to the extent he can) a degree of concern.
This gives my spouse hope that he would be able to teach Bil to check on his mother if he can make it part of Bil's routine.
Of course, I tend to be a little more pessimistic. Bil did not do any of this under his own volition. She wasn't wearing her medical alarm pendant, so if she had been knocked unconscious, or unable to speak, I still don't know if he would have called 911, or (this sounds harsh but this is how autism can be) if he would have waited until he was really hungry, or if his routine was otherwise somehow disrupted, before reacting.
My mother in law feels she doesn't have to wear her medical alarm if Bil is in the house. She has so much faith in him. I don't doubt they have a very special, almost symbiotic, relationship, But as to trusting that Bil could call 911 unassisted.....
I think that is wrong, and could have a tragic result.
But I am very, very glad that he is helping out under her direction. We are 3 hours away, in the middle of winter, there was a snowstorm last night, and we might not be able to get down right away there if, heaven forbid, something bad happened. I hope she uses his help as much as he can give it. I've been injured in the same way as she has been, and it was no picnic. She needs, and deserves, his help.