I am a single Dad (49) full-time dad to 14yr daughter Kate, 17 yr son Ben. 2 years after divorcing my narcissistic wife of 19 years. she showed all the classic signs of a narcissist, alienating my family and friends, constant arguing and abuse, never shows remorse, constant threats of suicide or divorce to have her own way. The arguments became increasingly violent towards me, and my son became suicidal and developed a food disorder.  She bit me badly one time, my son called police and that's how I managed to get custody and got the kids out of there. That was back in June 2019. Ben blossomed – and shows all the signs of an empath. My daughter Kate, who was regularly run-down by her mother both privately and in public has gone the other way as a narcissistic copy of her mother. She takes no responsibility for her actions, will bully and run-down others – supposedly friends – to get her own way. She has no empathy, claims everyone is against her, will hold grudges forever, argue everything, and threats of suicide are now common.

I have been in a very strong relationship of 1 ½ years with a woman who has a daughter  13yo (1 ½ years younger than Kate), Anna. We did not move in together yet because of my daughter.

Our daughters got on well as a mutual interest in horses bonded them until Anna grew tired of Kate actions of bossing\bullying\put downs.

Anna ended the friendship politely. Soon after, anonymous posts were made on social media to humiliate and discredit Anna.  This caused a huge upset, and we were certain Kate did it, as Anna has no enemies.
Anna asked not to make the situation worse by confronting Kate, as she was afraid it will escalate. So we never said anything.
Kate did a hoover to get Anna back and Anna complied as she was scared of more internet abuse.

It's a year since then. Anna feels trapped, and can’t escape the constant texts of Kate, the demands, the insults about her mum, the constant criticisms about everything she does. Kate even tells her off for seeing other friends. They hardly ever meet, but Kate is oblivious to this, sees them as “besties” yet will ignore things like Anna's birthday. Any opposition or "no" from Anna causes Kate's outrage that Anna has to calm.

Anna is never abusive and tries to calm things down with Kate also for the sake of her mum's relationship. As she feels she has no choice but to calm Kates bad moods and complaints. Anna is too scared to say anything as she knows she will be bullied online – particularly as Kate knows kids at her school.

My partner and I also want the “friendship” to end. The problem is that Anna doesn't want us to mention anything to Kate about how she feels for fear of reprisals. She thinks when Kate finds out we would not be able to stop the online abuse and she will suffer the consequences, not us. She thinks there is no way out. And even if we break up she will still hunt her.

My question: Anna is afraid to break "friendship" with Kate again because of possible Kate's revenge online (as she is very vengeful)  What can we do about it? How to end this friendship and set Anna free from Kate's control?

( names have been changed to preserve privacy)