No, you cannot earn points by buying someone else a ticket. Sorry.
Now, that said, maybe you'd get lucky with granddad using your number, but I wouldn't want to bet on it. And if you get caught, they could cancel your account and you would loose all of your current points.
I tend to doubt this - I've accidentally done this a few times, reason being I've made reservations to take a trip, only to have a friend later chime in "Yeah, I'll go - you know the dates and trains, so if you book it for me, I'm good for it."
So off to Amtrak.com I'd go, and I'd log in, and get the itinerary straight so it matches the existing one, and proceed to the purchase page, and there I'd see MY name already filled in - so I'd take care to delete that and type my riding companion's name in the passenger box, then proceed to enter the payment information, and complete the purchase.
When the confirmation arrives, I'd then realize that by logging in, Amtrak.com not only pre-populated my name info, but also my AGR number, even though the reservation is in someone else's name.
In the end, I never got any points for the couple of times this happened (and honestly, I shouldn't have) but I never got any message later accusing me of an unsavory practice. It was just something I overlooked while trying to look over several other fields of data to ensure they were correct.