
To be honest I think both of them thought it would be a trap
http://www.mangago.zone/read-manga/pumpkin_night/mdx/d_v-2-chapter-8/7/
but I don't think they knew that they would take it so far.

Several things.
When people are getting attacked sometimes people don't react because they are to scared or think someone else will intervene. It has been studied and is known as the Bystander effect. Literally when I was learning CPR & first aid we were taught to single out someone and order them to call 911 in and emergence rather then as the crowed to call 911 because of this.
Another thing we are taught in that in an emergency as much as we want to help a person if the scene is dangerous and it is out of our league of expertise it is better to call for help than to try. If we were to try and get injured yourself, the responders will have two victim on there hands instead of one. This sort of applies here. Yes he could have done something (and deep down I wish he did) but he is just one high school student in what seem like a class full of unsympathetic children/adults. I wish he would have done something but I don't entirely blame him unless I see evidence that he was actually in on the "joke", which so far it seems that her wasn't. The ones I really blame are the adults who literally covered up an attack because you don't prank someone using sulfuric acid.
Also by the looks of it Naoko hasn't spared him, she just for now saving him for last.

That's fine. In a way I don't either because just watching is fucked up to me too. But I also understand where he is coming from and don't wish him to die (even though there is a possibility that Naoko will still kill him. Your feelings are not wrong per se, I just wanted to present the other side for better understanding.
Am I the only one hoping that the boy joins her in seeking revenge? Cause that would be awesome