You introduced her to all your friends, got her a job where you work, introduced her to new brands and designers and introduced her to your big brother who is now her boyfriend.
Then she stabs you in the back. After you've made sure she never comes near you or your friends again, take the time to mourn for your pseudo-friendship, and then secretly thank her for being so nasty.
Thanks to her treachery, you've learned to be less naïve and you'll never make that mistake again with anyone else. You need to thank her for mistakes you'll be sure never to make again!