I have some strong suspicions about Jay that I won't go into in detail, but they seem to add up in my mind with the obsessions and the inability to recognise social cues. His mother coddling him like that isn't helping him at all.
Let's not diagnose someone based on a single interaction we heard about fourth-hand.
I think it's a valid point (which was also my impression), which is why I think it was important for the DD to be more clear in her response to him, that she was committed to a job and would not be changing her mind about going to prom. This comes back to life skills and communication skills that she will learn as she grows, but especially when dealing with individuals like this, you need to be more direct. The mother isn't doing him any favors if she's coddling his whims and not teaching him how to function in regular society. I think most of us who read this post, really didn't see where there was a question at all that DD would not be going to prom, but this boy was clearly mistaken because his brain doesn't necessarily comprehend nuances like this. Of course I don't know for a fact that this is the case with Jay. He may not necessarily have a disorder, but his thinking style and expectations seem very immature.