From my experience, you need very good boundaries and to know yourself a whole lot when you deal with someone with BPD.

I can only speak for myself, but sometimes I come into a session and I'm a complete and total mess. I can't even really talk about the issue, just sit there and cry, be scared, whatever comes up. My T says that this also provokes emotions in him. Now if the T knows about the disorder, they can judge what to do in this situation. But if not, the T might for example be too comforting, making me get sadder and sadder because that's how I get more comforting. And then at some point the T is exhausted and angry, snaps or tells me to leave. And then I've repeated the cycle I've been in my whole life.

If they don't know what the diagnosis means, how should they know how to treat it? If your doctor told you he didn't know what exactly cancer is, would you want him to treat your cancer?