I know she has it and for some reason doesn't have custody of her own child.
That says volumes. Most of the people on the Family Law board have shared custody with their BPD exes. It's really hard to lose complete custody. I wonder if there are any public files on her custody case that you could track down?
In terms of what how she might be around your D, it's possible that she will be jealous of your ex's attention on her. There is a child like quality to many BPD sufferers, and they have a hard time regulating their emotions.
You can read more about BPD traits and parenting here:
https://bpdfamily.com/message_board/index.php?topic=182254.msg1331289#msg1331289It's also possible that your ex's new BPD gf is in the idealization stage. It's often the devaluation stage when things start to worsen.
The book Turkish recommended is great -- it will help you create a strong bond with your D so that she feels emotionally validated when she talks to you about her experiences. It's an amazing tool for any parent, but in particular for kids who come into contact with BPD sufferers.