I think I might have quiet BPD, but I’ve only had really bad symptoms when I’ve been in romantic relationships. When I’m single, I’m a lot more stable; I still have issues relating to others etc, but the symptoms are more manageable/less evident. As amandalouise pointed out, everyone is affected differently by trauma, no matter what underlying conditions they might have. In my case it could be that early trauma helped lead to BPD, but it can probably be vice versa for others.
In any case, it’s still up to your ex to figure this out for themselves, for better or worse.