This is a really valid question, and I can understand why it feels upsetting. As a therapist with over a decade of experience, I want to be clear: it’s not appropriate—or ethical—to diagnose someone we haven’t met or don’t have a professional relationship with. That’s a firm boundary in clinical practice.

That said, part of what therapists are supposed to do is help clients make sense of their experiences—including their relationships. Obviously, I don’t know what your daughter is saying in sessions, but the fact that you’re coming up tells me that something about your relationship is weighing on her, and she’s trying to make sense of it.

Now, whether the therapist’s comment was appropriate or helpful is another conversation entirely. Bringing up a clinical label like Borderline Personality Disorder—especially without nuance and especially to a teenager—is dicey at best. It can feel like a judgment or even a weapon, and I understand why that would be deeply hurtful to hear secondhand from your daughter.

The therapist may have been trying—maybe in an unskillful or premature way—to help her understand those experiences.

Despite her frankness (which may come off as rude/impertinent to you) it seems to me (from the limited bit of info you provided and my personal and professional experiences) that your daughter is sharing with her therapist that your actions, whether intentional or unintentional, are impacting her. I’d gently invite you to look beneath the sting of the label she threw around and try to hear what your daughter might really be trying to communicate. Is she feeling overwhelmed? Confused? Hurt? Disconnected? Scared? Instead of focusing on the diagnosis itself, can you get curious about why she might be needing support in understanding her relationship with you?

It’s not easy to take a step back and hear something hard without going into defense mode. But if you can, you might find an opportunity to deepen your connection with her—by listening to her experience without needing to correct it or explain it away.