Oh... .

It depends too.
Staying->gets advice from people who've chosen to stay
Undecided->gets advice from people who're considering staying
Leaving->gets advice from people who are leaving or who have left
Overall, BPDs are fairly unpleasant people (One a scale of 1-10, with 1 being a serial killer, 2 being a malignant narcissist... .BPDs are roughly a 3.), and those R/S which work out tend to have people coming here less... .so advice you get is most likely slightly biased towards the leaving side. Mind you, most of the leaving advice is perfectly justified... .it's just that there is some hope if you choose to pursue a R/S. (I've actually chosen to stay so far... .

(If you do choose to stay, please check out a therapist as staying with or being involved with a BPD is a red flag for mental health issues.)
Generally though, people who are leaving have had the most success with going completely NC. Most BPDs are actually surprisingly good at surviving when they're on their own. OTOH, there is a suicide risk - but even under ideal care, there's a suicide risk. Assuming responsibility for someone else's choice (particularly for someone with BPD) tends to lead to a lot of suffering. Check out oceangirl26's postings on her exBF for examples.
More to the point, well, making sure he knows about the option of therapy would be a neighborly thing to do. Letting him take time from your daughter would not be a good choice.
--Argyle
*Mind you... .my wife is doing about as well as possibly could be expected... .is 6 mo into DBT therapy... .and our last conversation with the MC involved a discussion of why I am responsible for not stopping her from screaming abuse and throwing things. BPD is one of the worse possible diagnoses... .I never thought I'd find myself hoping that other people were right when they guessed my wife might be a violent paranoid schizophrenic. If you do choose to stay... .the best possible outcome includes about 3 years of therapy (with a reasonable chance of success). The odds of getting that outcome seem to be about 1 in 4.