There are many members here who have had "a divorced ex-spouse who is undiagnosed and untreated BPD" parent, myself included.
We've found that most acting-out disordered parents manage to avoid getting diagnoses. So you may have to
accept that typical reality. For example, I was in and out of my family court for custody issues during some 8 years and never once did the court or professionals around the court seem interested in diagnosing my ex — or either one of us. It was only at the last court decision where the court remarked my ex should get counseling — but stopped short of ordering it.
Overall, courts deal with the litigants as they are. Courts will not try to fix parents. Rather, they issue decisions and orders to limit some of the worst of the poor behaviors. And they often are willing to categorize much of the conflict as squabbles and emotions they hope/expect will fade once the divorce is final. That works with reasonably normal parents, not so much our stubborn cases. We would do well to
accept that. What does that mean? Courts and professionals around the court rely on and generally base decisions on
documented evidence. We would do well to do similarly, accumulate documentation with sufficient details that the professionals can use to make their decisions and recommendations. (Vague claims like "he always..." or "she always..." are legally meaningless and often ignored as "hearsay". Much of what is needed is all about
documenting sufficient facts.)
We've also learned that a common behavior of people with BPD (pwBPD) is an intense
Denial of responsibility for their behavior, hence their constant
Blaming and
Blame Shifting onto others, typically onto those closest to them (such as us). While they are in such extremes of Denial, it is exceedingly hard for professionals to treat them.
We have a
Tools and Skills Workshop board (link) that contains numerous articles on how to communicate better, set practical boundaries and much, much more. It should be very helpful over time to provide more education about this disorder, the behavior patterns and how to address them in more effective ways.
However, if you could include some examples then we could respond with focused options and strategies for your consideration.