I mean, what do you do in a drive through when you get to the window to pay and it's like 3x more expensive than you expected? Just drive away? I'm sure that's what people would argue, while completely ignoring that it's a massive social taboo and 99% of people would just pay because of social pressure.
Also, what happens if the place starts making the food after you order, but before you pay (so you can get your food more quickly and efficiently!). Have you committed theft, or defrauded the business by deciding not to pay? I mean probably not, but I'd hate to be the first person sued to test if it holds up in court.
Unfortunately, I see a police car in her immediate future.