Hi OP,
I'm no psychiatrist, but my DD has BPD and so do a number of her friends (via psych wards).
This sort of behaviour from the girl is very much like the behaviour of unmedicated BPD - I ran the details of your thread past DD (who is 18 and has been in treatment for about 5 years and is functioning well) and she said the behaviour was very like that of one of her (now ex) BPD friends who has untreated BPD.
DD was quite normal, but she suddenly went from a lovely little girl to a monster seemingly overnight. We had no idea what went wrong and I'm just telling you this because if this girl does have BPD and the parents don't know and she's unmedicated then they will have no idea how to handle her - other than to do exactly what she tells them to. Which might explain their current behaviour.
BPD affects relationships and people with BPD have a very painful fear of being abandoned.
With all of that said - there is no easy solution to this. BPD is very serious mental health disorder, and untreated it causes a lot of pain to everybody around the person - but even more so to the person themselves.
As an adult dealing with BPD I was strongly advised by my GP and DD's psychiatrist and psychologist to get my own therapist as it's so difficult to live with a person with BPD.
Whether she has BPD or not, I would suggest that you try to get your son in front of a psychologist to help him (as far as that is possible, I know not necessarily easy), as her behaviour is definitely very much outside the boundaries of normal.
If it is BPD then you really won't be able to manage this on your own, or by removing phones/blocking on social media. I can't remember the full details, but while the police weren't great as you've found, there is a mechanism, I think via an AVO, to force them into treatment if they break it.