I've heard of this sort of thing happening, but i've never experienced it myself. From what i understand, this is what you have to look forward to as long as your SO is involved with this other person. There is no cure, no medication for BPD and it is often accompanied by other conditions, including but not limited to bipolar and social anxiety.
If I were you, i'd look deep within myself and ask myself if this person is so worth being with that you would put up with basically being involved with the borderline metamour. Then make a choice based on your personal priorities.