Even if she calls and tells you all the details and says she wants help, you can suggest she get a new T and/or draw a boundary that you can't really help her in that way. It is condescending to think she cannot manage her mental health decisions and it is possibly encouraging a codependent relationship.

There is no way to know what is going on in that therapeutic relationship nor is it your place to know. Even if you were still together, it would not be your place to know but especially since she is your ex. His modality might be this way or she may not have BPD (for example she might have cPTSD which had a lot of the same symptoms). He might be building a Therapeutic Alliance before delving into any work. The Therapeutic Alliance is vital for anyone with a trauma history. You just can't know why he is taking this nor should you need to. She can and should make this decision for herself. If she feels he is helping her, great. If not, suggest she try someone new or talk to him about it to change his approach. I would highly urge you not to disempower her by trying to make this decision for her.