A good therapist should have a balance between validating clients and challenging problematic behaviors. Depending on what parts of therapy someone tells you about, it might sound much more like just validation though. Even if my T challenges something I'm doing, I'll still not feel that he didn't validate what I'm doing, he might just suggest a different approach. Also, assuming your ex really does have BPD, if there's any kind of idealization going on it might be hard to tell whether her experience really matches reality.

Does your ex think the therapy helps her, have you ever asked her? If she thinks it does, you have to learn to sit with your feelings of it maybe not being the best for her. If she's happy with it and feels like it helps, that's fine. You're saying they 'have not started any kind of therapy', but therapy is just talking to somebody. There is not real 'start', your ex goes there and she talks to someone who is a therapist, there is not much more to it, unless you're doing specific programs (and that's not something all therapists do).

If your ex expresses concern about therapy helping, then maybe you can suggest that she goes to shop around a bit, try out a few different therapists. You can also suggest that if she seems happy with her therapist, but don't be surprised if she doesn't go see someone else.

Honestly, your post sounds to me like she just wants to share her journey and experience of therapy with somebody who she's close to. Just because she tells you details about her therapy doesn't mean you need to help her get the best possible thing (in your opinion) from therapy.