Well boats are made of fiberglass and they get repaired every day. I've even heard that someone once repaired a body panel on a corvette. (what I'mt trying to say here is that the dealer is an idiot)
The thing that confuses me is that fiberglass doesn't really 'dent'. Chip, crack, yes. But generally not dent.
Did he perhaps mean 'aluminum' ? Because that can be slightly tricky to repair, but again it's done every day.