Hm...I suppose at least he found out the true nature of the girl he liked, so that he could sort of move on with the girl who, well, sort of teases him all the time, but probably actually likes him.
Considering that ending, well, I'd say he actually lucked out.
I had the same impression and thought that she knew all along what his crush was doing and planned for him to see it so that he would abandon his crush and take a liking in her instead...
I would have started dating Kisaragi-san and then tell the teachers that a sex club was operating out of the old school building after school after I verified they were there. So I would get the girl AND ruined the other's lives.
I personally wouldn't say this is "fendom"; rather Kisaragi is more of "an assertive girl". Still, she seems to have taken a liking to Kiriyama ... only a continuation might confirm where things may go from here.
Considering that ending, well, I'd say he actually lucked out.