People with BPD on the whole, suck.
The only treatment requires them to really take a good hard look at themselves and they simply refuse to nor can they bare to. Especially the more malicious ones.
Cluster B is a pretty wide ranging spectrum. From quiet borderlines, to Sammie Busharts, to cold hard fucking psychopaths, to Susan Schofields. Anyone on cluster B typically carries traits from other cluster B disorders, even if they're only diagnosed with one specifically. Sub types exist.
From the way Sarah has acted though, I don't think she does have BPD. I don't think she's even particularly on cluster B anyway. Not about to armchair psychiatrist, of course, but if she did have BPD I think we would have seen some solid sperging by now. Shiloh style.
I feel fucked up teenage years and resulting depression is Sarah's problem, not BPD, but BPD is the absolute perfect way to discredit someone making allegations of abuse toward you because they are quite widely known to be the ones who make false allegations. If this ever went to court, she should get a second opinion and see about getting the diagnosis revoked. If the second opinion changes anything, it would make her way more reliable and attractive as 'the victim' to a jury.
If the actual evidence never comes to fruition and all the symptoms of BPD are read out in defense of Greg and his actions, guilty could be a tough sell. I guess it's why the predators always choose their prey like this. It's why some people still shit on and don't believe Virginia Roberts from the Epstein case.
With regard to Billie, I feel the most sorrow for her. She was already doomed before she even met Greg. Social media is bullshit. That interview was the real Billie. And she is teetering. She is on the edge, she is completely broken and you could see it. You could also see she definitely isn't the sharpest tool in the box at all. Not having the intelligence to be able to properly process what has happened to you must be torture. Ayalla is a hero. Without her, I think Billie would have become part of the furniture at the swamp shack.