Actually, that may work. I had to do some digging to find it but Pebble IS compatibe with IOS (iPhone). So since Pebble works with iphone (as long is it has BlueToothe - which I imagine all phone do now) this should be what you are looking for.
However that link does not go into detail of HOW to set the message or who to send it to.
The other thing I noticed is it's not an "always on" thing - you have to remember to turn it on and if you shake it you have to push a button to confirm before sending? Noy practical in my book.
A more practical thing is :
Always on my default.
If you shake it, it should vibrate for 3 seconds
start a countdown timer of 30 seconds with the message like RecueME engage detected press a button to cancel accidental tigeering
If no button pressed at the end of 30 seconds, the message changes to No canellation detected - RescueMe sending Text alerts now.
Also what constitutes a 'Shake" most people swing their arms when walking (I know the OP's Brother-in-law is using a walker) so a shake need to be defined a little better.
OP - this could be what you need, and may work. I just see things differently when not much info is readily available (I play devil's Advocate in seeing what issues others may not find as a problem)
Good luck and hope your Brother-in-law's recovery gets steadily better