First off, I'd be wondering what in the hell would possess me to go through someone else's browser history.
However, if I did somehow find out, I don't think I'd freak out or react that much to it really. If she tells me or someone tells me in front of her, I'll probably say something like "eh, whatever" (since really, it is their business, if they don't want people asking too much about it, I won't prod).