My personal belief is that the reason that a person suffering from BPD always degrades a Non in front of other people is rooted in the need for the sufferer to always be right. In order for a BPD sufferer to always be right the Non, by definition, always has to be wrong, "Heads I win, tails you lose". This sounds like stating the obvious; but the obvious masks the motives and actions of the BPD sufferer.
In order to make a Non wrong all of the time, an air of mistrust between the Non and the people they regularly have contact with must be created. To do this the BPD sufferer has to control the people with whom the Non has contact and the perception of the Non by those people.
In order to do this the Non has to be isolated from their own friends, family and anything which gives them healthy support, confidence and individuality. To do this the Non has to become part of a controlled circle of 'friends', and the Non removed from their own circle of friends and pastimes.
Once the Non is part of this new group the person suffering from BPD can now start making this group believe that the Non is a bad person. In all likelihood the process will already have begun. Ideally this group will get to the point that it will start reading bad motives into everything the Non does.
As the Non no longer has any other reference point or support structure, they can, I believe, get to the point that they believe this version of events themselves.
A person suffering from BPD is incredibly egocentric, everything in their lives has to be geared towards them, and they always have to be right. By creating the above situation they achieve the following aims:
The first thing to do is get out of the relationship; you cannot do any of the things required to recover while still under this negative influence. You will not be given the freedom required. Second thing is to remember who you really are, and that you are not the person you have been painted as being.
Break the cycle you have been put into, get out and meet people, make new friends. Re-establish contact with old friends, do the things you did before you meet the person suffering from BPD or if that isn't possible go out and do new things.
In short you have to re-establish yourself as an individual and let people see who you really are. This undoes the damage that has done, and reverses the effects of your treatment. There is strong likelihood that even the people who have been poisoned against you will eventually see through the lies.
My BPD sufferer did exactly this to me, they slowly but surely isolated me from all of the things that made me who I was and am. Isolated me from my friends, stopped me doing the sport I loved with the people I had done it with for years and even tried to isolate me from my family. This was done over a period of years, and so subtly that I never actually noticed what was happening. Only now in retrospect can I see what happened.
I am several months down the track, and I have been lucky. I was able to move back to my family. I have some brilliant friends, who have provided me with the help and support I have needed to start rebuilding myself. In many cases they are people who I got cut off from, and they have welcomed me back into the fold with open arms.
I still have a long way to go, but I am also a long way down the road. The first step is always the hardest, but the sooner you make that first step the sooner you can start to feel the benefits of making it.