How did Lexi know Katherine?

I think maybe she called Katherine a "raging bitch" because after Stefan told her the story she decided to call her that.
I think if Stefan was my friend and told me the story I'd call Katherine a raging bitch too.

i.e. My friend told me a few months ago that she told the boy she liked that she liked him and the guy was a total douche. Then I wanted some blood, but as time passed we just call him all kinds of names.

I guess it's what happens when your friend gets hurt like Stefan did...

lexi doesnt knoe katherine at all. how i understood it was that stefan and lexi are BFF's, rite? so as stefan's BFF he would automatically confide in lexi about everything and share things with her. i know elena does this wit bonnie too. and he had that picture of katherine too, and i think when elena came over and left she went in his room holding that picture and was like, "what the hell".