As usual, I am not a lawyer, consult a legal professional before making any decisions based on this information.
Even though an old work like that is in the public domain, if it was played as a recording, whoever owns the rights to the recording still has a claim to copyright.
But even if it was played live and sung by the attendees, that doesn't necessarily mean the sheet music is public domain. Pianos from hundreds of years ago didn't play exactly like a modern piano, so there may have been an intermediary step where the original piece was modified to work on a modern piano when the sheet music went to print. The modifications to the sheet music could grant the person who sold the book a copyright.
Doesn't necessarily mean the person has a valid claim, but it's not impossible.