Damon basically (if not explicitly) spelled it out for Bonnie. He was there to teach Elena how to feed, and dumb frat boys were the lesson plan. So, I mean she did kinda have a heads up. Plus she kinda already did the best friend becomes vampire thing so...

IDK Maybe if the show gave Bonnie a plot that didn't involve every single one of her family members being murdered she'd be a little bit more fun ;)