People with Borderline are particularly prone to get involved with people with Narcissistic personality Disorder.
She (like all of us) needs to learn to identify Red Flags and times she's being manipulated; the Freedom Programme can help with this.
I have BPD traits and what was groundbreaking recently for me is to cut ties with someone with narcissistic traits, who was very manipulative and coercive.
He would play on my fear of abandonment (a BPD trait) to get me to do what he wanted.
She needs (speaking mostly from my own experience) to realise that it's not worth putting up with any old bollox due to avoid losing someone, esp. if the person is a crap person to have around anyway, who's just using her, using her for both an emotional response and the consequent ego boost and more.
It gave me a trememdous boost to my self esteem to get rid of the user, the player- affirming to myself that I was worth more.
EMDR therapy was also useful in getting at the underlying causes of low self esteem and lessening them.
Mumsnet helped tremendously in helping me realize what he was like when I made a thread.
It doesn't often get somewhere to tell someone their relationship is destructive, the person they're with a user, abusive etc, but it's worth a go.
I would advise that you say to her to make a thread here if she has doubts about a guy.
You can also play the same role yourself- when the bloke has done something to hurt her, tell her that the way he's treating her is not ok and she needs to cut contact etc.
IDK if it helped me when friends did that, but it didn't do any harm.
Best wishes. xxx
Oh and I did try DBT and it didn't really help me but it does for some people. It did help to see a therapist, just to affirm to myself that I was determined to look after myself and do better. Spending a lot of money on therapy meant I had to show myself I was doing better, or I'd wasted thousands of pounds (I'm not earning BTW- I spent my disability money on it. It was worth it.))
What I eventually found helpful in general was the EMDR (all BPD is based on childhood trauma of some kind- in my place mostly bullying/rejection at school, so EMDR can help as it addresses the trauma. I was really skeptical, but it's been excellent.)