I would insist on her finding another job as a precondition to saving the marriage. I can't answer your question about BPD, but I think in any relationship the 3rd person has to be fully removed from the equation. In the case of BPD, the next time she paints you black (and it will happen), you don't want her seeing her former lover in the office.
This is exactly what I was thinking would end up happening. She may have a promotion potential that would take her out of the office, but if that doesn't happen, I don't think she would be willing to quit her job and get a different one.
Things were better for a while, but he's escalating again, and saying all the things he said before last split up. It's like things will be okay when he has a good view of me, but slowly that starts to erode, and then I'm painted black, and he starts treating me like crap.
That is my biggest fear. I think that that is what she would probably do too... .because she is queen of the flaky people. She gets all excited about something new and then abandons it a few days or weeks later.
As unfortunate as it is to think about, I think that it may be time to start throwing in the towel here.