Hi dmx,
the guys have pretty much covered it (esp the owls). The issue is with her, not necessarily with you. The trick is to be consistent I would say. Accept that sometimes she's going to throw a wobbly with you, and other times she will be fine. It's not that you've consciously done anything to upset her - you're just the nearest target when she's having a particularly bad day/few days/minute/hour/whatever. People with BPD tend to flit between 5 different "modes" - healthy (not very often unless you're in a recovery period), vulnerable child (which is horrendous so we avoid it like the plague), angry child (we can't handle the vulnerable feelings so we lash out), detached protector (kinda numb and dissociated from everything to avoid feeling any kind of emotion) and punitive parent (the self-critical voice that constantly tells us we could be doing better).
When there have been a lot of triggers all at once, it's very easy to flip into the angry child mode and put the blame on other people for how bad we're feeling inside. it's not rational, and it's not pretty but it's 100% pure emotion and pain. So if your friend does start flipping out, say something like "I know you're feeling hurt and upset about something and the emotional pain is difficult. I don't want to fall out with you because I know this is the BPD talking, not you and I'm not upset by what you've said. I'm going to leave just now and give you time to calm down. But I'll phone you in an hour to see how you're doing and I can always come back round then."
If someone said that to me, I would probably burst into tears and sob apologetically. If I were really annoyed, it might help me cool down more quickly and realise WHY I was in angry mode. Just make sure she's not likely to harm herself - if she threatens to self-harm if you leave then it gets more difficult. But I think, if it were me, the fact that you say you're not upset and promise to talk in a short time, that would be enough to get me through it. Just remember to say you're not upset and you ARE coming back. Hope that helps.