None of the people acted reasonably or responsibly.
That pretty much sums up this case.
This was all over difference of about $2.30.
I have a feeling when this woman asked to see the manager, she was less than calm and collected (especially in light of the fact that she ended up poring her drink out all over the countet). Odds are, she acted like a jerk and the manager decided not to cut her any slack. In most situations, people who remain calm and rational in customer service situations tend to get better results.