I don't care how shitty a customer is, you do NOT mess with their food. Ever. I've known people who ended up hospitalized because of this. Those people were not being rude either. One of them was a girl who asked for decaf because caffeine can put her in the hospital. The barista gave her regular coffee though because they thought she was just on a fad diet, and she ended up in the ER. Whether it's a preference or a dietary restriction, don't EVER, ever mess with someone's food. It's not okay.