Thanks to all who provided a script for dealing with the Collection Agency issue I wrote about. I was going to call the dentist to recount the story today, but did not have the energy. Good thing, because I had some of the facts wrong.
Mr FS called the office to pay. He did so, by giving our card number. AFTER he paid, the receptionist/manager said "I sent this to a collection agency. I will try to get the extra fee removed." (!!!!!) So--she took the payment after the bill had been sent to the Collection Agency.
I had assumed that Mr FS had not paid BECAUSE we were told the bill had been sent to the Collection Agency. I am hoping the receptionist tried to get the bill back. But I am concerned because we are entirely without leverage of any sort now.
I will use some of the scripts and advice provided by my most helpful commenters. Many, many thanks. Any additional tips or advice?