Who was the expert at dealing with dumb collections people on a billing mistake?

I'm not super great on exact details, but essentially was seen in Oct 2016, had initial payment and settled it on a second payment from a doctors office. Company was bought out midway thru this year and their new billing folks issued her a second bill for piddly $80 some dollar thing.... she called them, they said oh they see where she had made her 2 payments and she should owe nothing. Next month she gets a notice from collections. She's called a few times with the Doctors billing folk and now has a saved voicemail saying she didn't owe anything as well as an email stating stuff was in error. (Think it sounds like part of the bill was double billed?)

And then she's still getting after all that emails coming back trying to say that the charges are correct after previously stating things were double billed and payments had been made. She's losing her mind and I recall some folk here with a post a few years back about dealing with retards in billing/collections. (Though this seems really on the billing tards than collections, myself have a office bill that's been in insurance limbo for a fking year too)