Marine Cpl
.577 Tyrannosaur.
This ^^^Everything was fine up until the gun dealer called the cops. Of course he could have bought it, or just told her to continue on. He's not bound to do anything in this situation. He chose to.
Even is there was a cop in the store at the front door working security, the FFL has no obligations to enforce laws by calling the police.
It's no different then when they catch a shoplifter and let him go with a stern warning to never come back rather than call the cops and lose business.