To some, it may appear that she was hired as a 'friend' when in fact, she was highly recommended (with right skills and experience) by the OP herself! So, friend or no friend, conspiring behind your manager's back with another subordinate is not the way to go, much less if that manager happens to be the one that got you hired!
Emma, your so called 'friend/employee' was never a good fit. Why? Because she behaved unprofessionally. She may not have to be your friend in the workplace, but she does owe loyalty to the company she works for.
A Counselor's role is to listen and to mediate in everyone's best interest (presumably) to avoid negative workplace repercussions and potential litigation.
She's not supposed to 'draft' adversarial letters for aggrieved employees against her own manager. You could file a complaint with HR, but such things tend to go sideways and there are never any winners.
I would put my own personal feelings aside. Why? Because unfortunately you now have to work with her. Best to play nice and let this one go for now. Barring more blatant misbehavior, be on your guard until her assignment runs dry or your own career takes you to better places (whichever comes first).