I'm gonna get lynched for this but...
While everyone is dumbfounded by the ignorance of the restaurant manager, there's one thing that isn't shown in the movie nor in the actual footage of the incident and that's the demand that a restaurant manager is under during peak meal periods at a high volume store. I can tell you from personal experience that you have to make split-second decisions in order to handle 101 requests for your attention every hour of the meal period. You have to watch a crew of young, minimum wage earning employees to make sure they don't need to be told the simplest things, get change for them from the safe, use your keys to unlock their register, handle order errors, do a walkthrough of the dining area, restrooms, and kitchen every 10 minutes, watching for secret shoppers who are crucial to your company ranking and performance, all while projecting an air of positive energy and authority. You saw how (in the movie) the manager had to shuffle around her entire staff to get one kid to watch the girl and even tried getting a non-employee who she knew well and thought she could trust to help.
Now there's no doubt that she made some really dumb decisions but consider the millions of things that you don't see happening in the restaurant that she was trying to deal with and it may shed a little light on the events that occurred in the office.