The crown is nickle, the glue that was holding it is the most toxic and the yuky tase he smelled/tasted when it came off was the deadliest nest of bacteria known to mankind that was producing trillions of babies to be swallowed in his saliva.
THE CROWN will not get stuck, it will pass out like normal food. Most people when eating nuts will swallow chunks as large as crowns and most people swallow food chunks much larger than crowns.
"IF" he wanted to save it and you knew you swallowed it, you walk outside and stick your finger to your throat and PUKE! I had to do this when my mouth was loaded with crowns.
Otherwise, now he has to check each bowel movement............I suggest all crowns are bad news and time to remove the root and have a blank spot to remind him for the rest of his life of his health condition. Naturally people won't go to this extreme and seek to replace their crown.