I have a slightly different perspective...
I would say, first are you her lover too? If not, why are you stepping into her meltdowns? I understand if yr SO wants you to listen to him complain, but if YOU are directly involved with her, it sounds like a bit of co-depepence to me...Unless you are a solid 3, not a V, why are her problems your problems?
Second, I am reading A LOT of snap judgements here. From what I have read about BPD it's an ephemeral "illness' that can apply to a lot of people. Are all people emotionally stable? Not moody? Not afraid of being abandoned? Further, it sounds like (to me) that someone gets fucked by being abandoned by their parents, then they get fucked by being labeled and hated by everyone.. what fun!
I can totally understand that you might find this all tedious with her,,,and want a calmer life. My thoughts would be step away. Let SO have the relationship - NOT you. And if you know she's afraid of being abandoned, why not assure that instead of fight it?
Hope I'm not out of line, but I'm bothered that people jumped all over her when you didn't even describe any of her behaviors....people are like, O she's BPD she must be list THIS and to me, that's a dangerous stereotype.