Because she wasn't actually an idiot, no matter how much he might think she was one right now.

"You weren't around," Karina said flatly. "Kathy didn't even show up for class that week and that meant I was in charge, since I'm one of your TAs. That meant it was up to me to do your job and check things out to make sure everyone else would be safe when we went over. So I went alone because it didn't occur to me to take one of the people I was supposed to be looking out for along with."