Your state does not require a written estimate for repairs unless requested, so it appears that the dealer has the right to bill you based upon your oral request for and authorization of the repairs. It's unfortunate that you assumed that the repair was free, and did not ask whether that would be the case.

He is taking the position that the damage was not there at the time your girlfriend purchased the truck. Your girlfriend is free to take a contrary position, and can even sue over it in small claims court, but that won't secure the return of the truck. As she bears the burden of proof in a lawsuit, to prevail in court your girlfriend will have to establish by a preponderance of the evidence either that the damage was present at the time she purchased the truck, or that the damage could not have occurred after she purchased the truck (essentially trying to prove pre-existing damage by implication).

If she pays by credit card, your girlfriend can dispute the charge. I can't promise that a dispute would succeed. If it were me, I would make a note that I was paying under protest on anything I had to sign associated to get the car returned by the dealer.