I'm so sorry about your psychologist. It sounds like that hurt a lot. Was there a reason for that? Or did they move or close their practice?
That's a tough question you have. I'm going to say that it depends on the friend. I have seen it both ways. On the one hand, I have seen how such knowledge can stigmatize a person. On the other hand, from personal experience, I wish I had known that a friend was likely BPD. I certainly would have behaved differently had I know and been able to research what he was going through. I think you will have to evaluate how she will react. I think you also have to consider how much BPD affects you. I have met some people for whom it was fairly mild and they interacted well and I have met others for whom it was socially debilitating and they were unable to have deep relationships as their symptoms overwhelmed them.
I hope it works out for you and that you find the right answer for yourself.