I dated a girl with a probable cluster b personality disorder (suspected histrionic) for four years and remained friends for two years thereafter. She was my most thorough and violent abuser. My father also likely has a cluster b personality disorder (narcissistic). Now that I've moved out our relationship is better but my parents' relationship is tragic.
If your hunch is correct and your best friend has a disorder from that group, trust me: YOU WILL NOT CHANGE HER. These disorders are egosyntonic: her behaviors do not distress HER and more than likely she genuinely believes the problem is with everyone else, not her actions or behaviors.
These disorders are notoriously difficult to treat and you are in no position to treat them.
it seems extremely unethical to turn around and tell the abuser that their victim is speaking out.
Yes. You can support the victim and urge them to protect themself but if you communicate to your best friend that you think she's being abusive it will at best accomplish nothing and at worst further isolate and endanger the victim.
I once told my dad that he needs to treat my mom better. He seemed shocked and asserted that he doesn't treat her poorly. He firmly believes that by supporting her financially (read: allow her to live in his house while controlling and mystifying her own money) that that means he's good to her, and that his constant verbal and emotional abuse isn't hurtful. I mean, narcissists really don't get other people's emotions. I may as well have suggested a blind man look harder.
I avoid fighting with my dad only because I know he'll turn around and dump on my mom in return. If it were 1v1 it'd be different. It's not.
By the way, the girlfriend I mentioned in the beginning dated my (male) best friend after me. She physically assaulted him in public and still managed to play victim and gain sympathizers who thought my best friend was harassing her even though she was hitting and slapping him in the face. On the street. Drunk.
You know what stopped her? He dumped her. And then we commiserated over the abuses she inflicted on the both of us.
My parents have been married for over thirty years. My dad hasn't changed. He's been in family and personal therapy, by the way. It didn't help.
Only my mom can choose to leave my dad. I've advised her to do so since I was nine, two decades ago. Nothing I've said or done has broken through to my dad.
I get where you're coming from. In my experience, there isn't much you can do.