Dart,
I rather suspect that he'd have to trade in the tickets eventually. But that is the key word, eventually. He could do it on the day of departure, as opposed to having to mail the tickets back. He should call Amtrak and discuss this with them, but I'm pretty sure that they can process things to get him the discounted rate without needing the tickets back right away. Again, I suspect that he'll have to visit a ticket window befor departure to exchange, although perhaps no exchange will be needed. I'm just not sure.
He is boarding in LAP, nearest stations for tickets are KCY & STL and they live halfway between the two. If they could exchange tickets with the conductor when boarding might be worth calling, but otherwise it is only a $22 savings since the discount only applies to the railfare ($151) correct?
At least we will know for next time, and hopefully there will be one, it is their very first Amtrak trip.