Yes the kid was disrespectful, and he should be fired. I believe the manager did the right thing by keeping her mouth shut, considering he's recording the encounter. She could have said something, anything, not even offensive, but something that could have been used against the franchise or corporation in some BS law suit.
Nobody is wondering what the agenda was here? Who pulls up in a drive through with the intention of recording something wrong? It wasn't just about the fries or the money, he had some other ulterior motive to be ready with his recording.