surely you can understand how the person would feel if they expected a date but got something else?
I can tell you exactly what I would do if I went on a date with a girl and she brought a friend. I would introduce myself to both and be happy I got to meet two people to potentially befriend and not just one. Because that's what first dates are often for, you befriend someone and see if you want to spend more time together.
But if their agreement was a date, that is usually between two people and bringing with a third party is not something you do.
I don't care what people are "supposed" to do. I'm almost allergic to people telling me "that's not what you do". I'm me, she is she and if that guy wants to date her he should date her and not some abstract societal expectation.
I'm not defending the petty response to this btw. It is very unnecessary and they could just go home, but this doesn't remove possible disappointment.
Ok so you had a bad first date, call big whoop magazine and see if it gets published...