I think maybe it was one of Henry's parents? But I am only guessing at that, because Parent X did not have a student traveling with the team to South Carolina. But even if I'm right, I don't understand why that revelation was so important and meaningful to Affenlight. And if I'm wrong, I really REALLY don't understand!!
911gal, I thought the same thing for the same reason, but agree that it didn't seem to explain Affenlight's stunned reaction (or his resulting thought that the world worked "always through particular people"), especially since Henry's parents had been largely absent since chapter 2. That's what made me think I must be wrong. I hope someone else can help clear this up.
Yes, it's Henry's parents. I think Affenlight just has such a reaction because they were attempting to conceal the parents, but there is only one person who is not traveling with the team at that point, and that's Henry. So, he realizes the truth of who "told on him." Also, if you think back to the beginning, Henry's parents were so upset that Owen is gay, so then I find it even more compelling that they would be moved to mention something about Affenlight and Owen at the motel.
It wasn't Henry's Parents. They are in town with Sophie, who had walked from their hotel to Henry's dorm. It was David who was on his way back to Chicago after Pella has made a firm break with him. Also remember, Guert, resented David for as taking his young daughter away from him, but here is Guert now doing the same thing with a young person. In both cases the younger person had wanted sex with the older person, so Guert's Haa Haa Moment is the realization that he has been betrayed by the man who hates him. The school officials are telling half truths. It is not a parent who betrayed Guert, but it is a person who does not have a son on the team. Also, David is the only really character in the book who the reader doesn't like. David is just creepy.