I have worked with numerous clients with B.P.D and I must advise you of certain things you need to do to protect yourself. Firstly if you are working alone with this client You need clinical supervision.
Secondly Boundaries are everything in this relationship. Set out clearly on the onset the agreed treatment agenda. Make sure that you identify what you will do and what you will not do. If the client breaks the boundary (which is usually inevitable) then ensure that consequences are put into place (i.e terminate the session there and then or discharge).
Is this client coming for hypnotherapy to deal with aspects of BPD? I reccomend that you read all you can about this extremely tricky therapeutic meliux. Irvine Yallom gives wonderful accounts of typical BPD behaviour in his book "an introduction to groupwork" (ok I know its about group dynamics but this really doesnt make much difference in this account).
I also reccommend another exellent book on the subject called "Self management of borderline personality disorder.A therapist guided approach" Langley 1993.
If you need to talk more about it then contact me via