Hi lostandconfused,
That's a good question, let's look at the possible causes for BPD.
Genetics can come into play. Environment can be another one, it can determine how your neural pathways are developed with what you're exposed to. A neural pathway is like a superhighway that nerve cells travel through in your brain, let's say that you go to a comfort food to help you sooth, if you keep repeating this habit the more solid the superhighway is, DBT can help pwBPDchange these neural pathways. A traumatic event can also be the catalyst for developing BPD.
I think that if your ex was exposed to another chaotic, intensely emotional pwBPD it would probably make her emotionally dysregulated. A pwBPD can't self sooth or regulate their emotions and it takes them much much longer to return to baseline than a non, i can see how she wouldn't catch a break and be emotionally dysregulated for long periods.