ARRRG. Yes, it sounds like it could be a difficult conversation.
Both of you have been affected.  :)oes her son know about BPD? If not, the book about growing up with a borderline mother might be of use to him, when you think it might be appropriate to mention.
But for you, the conversation will be a trigger. Keep your boundaries. Perhaps let her son know that the conversation has to be about the relationship between you and him. And you need for him to respect your boundaries in order to interact. He's old enough to be expected to respect such a request. And I'd bet it would be good for him to let go of the care taking.
And maybe ahead of time, think about and write down a list of ok topics to talk about, and topics not on the table. Think about what you are willing to discuss, and things that you are not, and gently start to establish those points. It can be a positive exercise in boundary development, as well as developing connections that do not revolve around the dominant theme of the past... . the ex.
Maybe, think about it like an agenda for important negotiations in the Majority Whip's office. There are salient points of discussion, which have been agreed to in advance, and other topics are not for that particular meeting because it will be nonproductive.
Frank Underwood wouldn't be swayed outside of his agenda
Good luck.