She sent a very clear message to the customer that his shitty, entitled, and borderline threatening behavior will not be tolerated. Seems to me like she made the world a slightly better place.
Calling it “inert” is misleading. It’s involved in all kinds of chemical reactions that are essential for life (and lots of non-biological reactions, too). It’s only inert in the sense that most living things can’t use it directly from the air and rely on nitrogen-fixing plants and bacteria to make it into molecules we can use.
A few microwaves don’t do that because they’re able to continuously run the magnetron at less than full power by using an inverter instead of a transformer.