I have a friend with a severe dairy allergy. We joke about how “high maintenance” she is when she orders her meals at restaurants, but it’s imperative that her food is cooked without butter, cheese, and other dairy products or she’ll get violently ill.
Years ago, she went to dinner to meet her boyfriend’s parents (now her in-laws) at a fancy Italian restaurant. She went through her usual ordering routine, alerting the server that even a tiny amount of dairy would sicken her. When her meal was served, the pasta was covered in cheese, and she had to send it back, which was mortifying when she was trying to make a good impression on her future in-laws. She was lucky, though, that the mistake was so clearly visible. If the dish had been cooked in butter, or if a tiny amount of cream was added to the sauce, she wouldn’t have noticed the error and she would have gotten sick from eating it.
Whether a customer orders a meal as is, or requests a modification because of a life-threatening food allergy, because they’re a vegan, or just because they dislike cilantro, the restaurant should provide exactly the meal they wanted.