Sabotaging Friends?! ‘Dear Keke and Kat’

Our first 'Dear Keke and Kat' was yesterday and I can't say that I've been in this situation myself, but I can see how it could happen. Your best friend gets a new best friend and that girl is totally trying to sabotage! The. Worst.

I copied her email over to notes to help keep her anonymous.. but here's the gist. What would you do in this situation? Have you had this happen to you or someone you know?