I’m sure this is very, very frustrating. The fact is that your mother is an adult and, as such, she has the right to confidentiality with her therapist. Sadly, you can’t know if what she is telling you about her therapist’s opinions are the truth or if she is shoring up her own angry ideas by attributing them to her therapist.
It would not be unusual for a supervisor to tell a therapist to stay out of a family argument while he tries to help his patient sort through her own issues. Think of it this way: If he comes down on your side in the argument (or if your mother even interprets it that he is doing so), he’s lost your mother’s trust and has no way to help her. Before you conclude that the therapist is being inappropriate, I suggest you give it a little time. See whether your mother starts shifting in her behavior and thinking.
Meanwhile, you could either voicemail or send the therapist a note saying that you understand the rules of confidentiality so you won’t push for a meeting. You could also say that you hope he understands that you very much want a relationship with your mother and that part of her illness seems to be a need to present herself as a victim. Let him know that you are open to a family meeting when he thinks your mom is ready.
Then, I’m afraid, all you can do is be patient. The good sign is that your mother is going to see a therapist at all. On some level she knows she needs help. Hopefully, the work in therapy is going to pay off and you’ll finally have the relationship with her you’d like to have.
I wish you well.