We have had horses that had to be floated every 6 months, so I wouldn't be so quick to say your friend can't be trusted.
I got a horse that had just came back from the vet when I picked him up from being floated, he was still a bit groggy. I saw the vet bill, and talked to the vet myself about this teeth. When I got him home, I took him to my vet for a check up. He looked at his mouth and said, we need to fix his teeth pretty quickly. . When we sedated him, one of his top molars had grown completely down into the missing space below it in the bottom jaw, yet the vet that had just done his teeth did nothing about it. We fixed it up, but this horse had to have his teeth floated every 5-6 months because the overgrown tooth, even ground down to almost normal, always overgrew since it had the space below it, so it threw off the rest of his mouth.
So, one vet didn't do anything about it, my vet, and myself immediately saw a problem. Maybe the vet who floated this guys teeth missed something, or this horse might have a mouth issue that requires more dental work than normal.