Okay, I'm going to be very careful writing this, because it is not an easy subject for me. My ex almost certainly suffered from BPD - I say almost certainly, because she ticked every box, but every time she got close to a diagnosis she announced that there was nothing wrong with her (the problem was always everyone else) and withdrew from any intervention. Every person with BPD is different, and there are very different levels of severity in what sufferers experience. So I can only give advice on a general level and based on my own experience.
I understand the desire to help your friend. But my advice to you is to be very, very cautious. There is no doubt that those with BPD are in considerable emotional pain. Their behaviour comes from a place of extreme distress. I have every sympathy with that. However, their coping mechanisms are extremely dysfunctional. They can be quick to take offence - any perceived slight triggers deep-seated fears of abandonment. You may find that they experience things completely differently to you - a simple chat or interaction that you thought was entirely unremarkable, can trigger in them a defence mechanism at some perceived slight. In reliving something that happened, you may feel that they are describing something a million miles from what you recognise. Persuading them otherwise takes hours of painstaking work, and you may never get there. I never did work out the degree to which this was because her memory and sense of what happened was genuinely maladapted, and to what extent it was actually just gaslighting - because it always seemed to trigger a terrible rage, that would put me on the back foot, trying to reconcile what the hell I was being accused of this time against my (completely different) perception of the event. And somehow the outcome was always a tightening of her control, a further restriction of my freedoms, a distancing from my friends or family - whatever it took to placate her. Over a period of years, it is deeply damaging to be on the receiving end of this - you can totally forget what normal is.
The rages were terrifying. I use that word deliberately. It is three years since I divorced her, and she still has the capacity to trigger in me abject fear. And I'm a successful, professional guy who loves a good debate, climbs mountains for fun, and generally fears little. But I fear her. She can reduce me to a quivering wreck, even now.
The manipulative thing is real. Sorry to those who are reading this and have BPD, but the stereotype is there for a reason. It comes from a place of fear - I genuinely don't believe she ever woke up wondering how she could fuck with me today. But the coping strategies present with the disorder are dysfunctional - some have likened BPD to living in an emotional condition that never matured beyond that of a toddler. I recognise that. She knew what she wanted, to give her the sense of security she needed. And she would do anything to achieve it. Anything. Trouble is, it was never enough. There was a gaping void where her sense of self should be, that could never be filled.
Eventually, she became violent. That's when I quit.
Over the years, she did a lot of damage to me. And a lot of damage to our children. I don't think she meant to. She just didn't know how not to.
If you want to commit to helping your friend, that's admirable, and I wish you luck. But please protect yourself. Set iron-clad boundaries, because your friend will probably trample over boundaries if you let her. Trust your own perceptions, and do not allow yourself to be persuaded that things you know to be true are not true. Or vice versa. Understand that, no matter what you do for her, there may come a time when she turns on you. You may not understand why.
But know also that, when she's in a good place, she could be one of the loveliest people you ever meet. When she's healthy, she may display a sense of caring that you don't often find.
But that's when she's in a good place. Only you can decide whether the positives of the friendship outweigh the negatives. In my marriage, they didn't. It became a living hell. I have never been so happy as when I got my freedom back. Speaking for myself, I will never allow BPD back into my life again. Others reach different conclusions.
I'm sure there will be BPD sufferers reading this who will want to tell me I'm wrong. So I reiterate that this is my experience of living with someone who almost certainly had BPD. I'm not going to engage in any debate with BPD sufferers who disagree with me. This is my story. It is, as it is.