I am very eager to see answers on this thread due to a similar thing in my own life.
A particular person pushed herself into my "Geek group" and is apparently the bee's knees (heh-I'm only 26 and I love that phrase). I was great with that, always nice to get another gal in a group, our kids got along...Anyway, she pulled a bunch of moves, some that I posted about in the Special Snowflakes thread, so DH and I started to withdraw socially; then on Facebook she started to offend us quite a bit with excessively grating judgements on specific people and beliefs. It took some "don't want to rock the boat" time, but we both unfriended her.
She never asked why. She seemed to take it as it was meant; we weren't invited to kids' birthday parties (though I'd have had to decline after the SS treatment last time), they didn't call, and we didn't see each other except in large group settings.
At a recent group picnic she was running a conversation, totally ignoring and speaking over me and my opinions. My husband started having a side conversation with people he sat next to, she'd butt in on them with her comments to his companions, he would look at her until she stopped for a breath then he continued his sentence from the word she interrupted.
Yes, she seriously offended him, how did you know?
OP, you may have to deal with this in person, but until Katie contacts you, I wouldn't worry too much about it. She may be so offended she'll decide to ignore you and the defriending the way my SS did. But if she goes all dramatic to mutual friends, they'll see who the drama queen is. If she gets really mean about you they'll see who the meanie is. If she only complains to people you don't know, then it doesn't really matter.