Well it's a lesson learned for your DD really @Bearaque .
When you work in customer service people will look for the slightest thing to complain about, many will embellish what actually happened to make it sound worse and some will totally make stuff up. What you don't do is freely hand them the ammunition to complain about you if you can at all help it! And younger staff with less life experience need to be guided and taught that.
And that's unfortunately what your DD and her colleague did. Most people wouldn't be happy with this type of service and although may not go to the management over it, would certainly have a grumble and some thoughts on it.
Unfortunately many customers are very entitled and rude and see people serving them, and especially young people, as an outlet for that. I'm the first defend our staff and step in and make it clear that unless they treat our staff with a basic level of respect, they're not welcome.
But in this instance, where DD and the colleague admit that it happened then I would speak to them and explain where they went wrong, why and how to address the situation in the future. If that upsets them, well that's unfortunate but it does need to be addressed because if they carry on then so will the complaints.
It's not the end of the world, the customer hasn't come to any harm, however some customers will behave like it's been a traumatic and life changing event and feel justified in doing so, it's not and it's a ridiculous over reaction, however it is the reality of working in a public facing role, no matter what it is or what age you are.
I'm not sure you'd have been happy to be served that way, most people wouldn't, a lot wouldn't go to the manager over it but they'd have thoughts and a grumble.
You feel the way you do because your DD is upset, totally understandable, but don't try and justify it by saying the customer was the one in the wrong and should have just been happy to get their food, because that's not what's expected by the vast majority of people when they're a customer.
So many people are expectant of great service wherever they go, complain about poor service, will leave 'honest' reviews, will complain about the slightest little thing - until it's them or a loved one being expected to deliver, then it's all not fair and expectations are too high.