Her anger is understandable, and her gut emotional sense that the store shouldn't be allowed to pull what they did was justifiable and furthermore correct. She knew bullshit and injustice when she saw it. She just didn't know how to correctly work the system to get the problem resolved, and her mistake was to default to angrily calling 911, and now she's in trouble and humiliated on national TV for it.
The correct solution is not to call 911, but to walk away furious, and go home. Most people would stop at walking away furious and going home, and that works, but then you'd have to fume over the fact that you did, in fact, get screwed out of your money. But you can take it further. Once you get home, write an irate letter to McDonalds Corporate, naming the specific store and employees you had trouble with. McDonalds Corporate will then handle the situation: they'll reply with an apology letter, giving you the satisfaction of having the fact that you were right acknowledged by the very company that screwed you over earlier. They'll go further and offer material compensation in the form of certificates for free food, so that you're no longer screwed out of your dollar or two, and actually are up a couple bucks. And finally, they'll quietly bring up the issue with the store in question, chastising them for their poor customer service and instructing them on the correct procedure for how to deal with being out of chicken nuggets. On all counts, you win.
Calling 911 was the wrong solution. She just didn't know the right one, and refused to believe the world was so unjust that she couldn't win this battle she rightfully should have.