Very important is that the follow-up sessions don't get stopped. BPD includes perspectives where the person has immense Denial and Blame Shifting of how/why they contributed to the situation. So the sessions may start strong but then the old patterns return. Sorry, Borderline is not something that has a quick fix. It will take months and probably years to address the unhealthy patterns of perceptions and thinking.
If the person was not informed of a BPD diagnosis, then let the professionals handle that information. You as a close person could trigger a lot of emotional "baggage" (not your fault!) that could surge up in that person's mind whereas therapy might be accepted better from an emotionally neutral professional.
Boundaries are very important. While there may be times to make occasional exceptions, strong boundaries reduce risk of the other person relentlessly chipping away at them. What do we mean by boundaries?
Have you read Henry Cloud's Boundaries?
Boundaries are for you, not her. You already know you can't tell her what to do or not do. You can't force her to do or not do something, your power is in your response. However, what can and does work (though there are limits) is something like this... .
"If you do or don't do ___ then I will do or not do ___."
Examples:
If you start blocking me from our kids... .
... .then I will enforce the parenting schedule, in court if that's what it takes.
If you want extra time for ___... .
... .then I may allow it but with a trade for equivalent time for ___.
When done right "if... .then... ." is powerful. It took me years to figure how to make boundaries such as these.
Oh, and since this would be a change to your behavior pattern, expect her to flame out with extinction bursts in attempts to make you retreat back into prior compliant, appeasing actions. She may never fully accept that you will run your own life, but in time she ought to realize you're not acquiescing to her demands as before and not push your boundaries as relentlessly.
Lastly, since you didn't describe other factors in your relationship, I'll encourage you to save the documentation of his/her participation in that program. Most people, and especially people with BPD (pwBPD), want to hide past problems. Fine, except... you may need this information in the years to come. What if he/she at some future time accuses you of DV, the documented history of mental health issues may help you defend yourself. Similarly if there is a divorce or if there are children and a custody struggle arises.
