Hi JerryRG,
I can understand why we'd want to defend our character with friends and family, BPD is a persecution complex and the person believes that they're problems are caused by outside circumstances.
It's not easy, but it gets easier with practice, it also helps to read as much as you can on BPD psychopathology to depersonalize the behaviors. Your ex is going to blame shift if she's reamed out by the mother, it keeps the drama going with a drama triangle.
If we move to the center if the triangle that way we have elements of all the corners in the triangle with rescuer, persecutor and victim. If we don't take either side with the mother and ex the payoff is our emotional well being, self protection, keeps our values intact and it helps with our detachment.
Escaping Conflict and the Karpman Drama Triangle