Flight attendant seems to be on a power trip : “...I will turn this plane around ...” She does not have that authority but she knows that nine and a half times out of ten the Captain will back up the cabin (flight attendant) crew.
While almost all First class passengers are on board with their carry on baggage stowed by the time general boarding,
passengers should follow crew instructions.
First class may be allowed to board early and get carry-on stowed, sometimes we have tight schedules and find the bins already loaded by others. Very frustrating. The passenger was in the wrong, attendant perhaps could have been more diplomatic, but the passenger escalated the dispute.