I only realised when my friend started talking about her therapist and mentioned her name. Wow it was a big surprise, and not pleasant because of my issues.

I informed my therapist and my therapist checked with my friend. It was a surprise to my therapist and that friend too as my social name is different from my legal name. (My therapist knows both of my names but friends usually don't know my legal name).

Her rationale is...I don't remember but it didn't make much sense. I didn't want to hear my friend talking about her therapy either way due to my own issues, but I don't recall my therapist giving clear reasons.

We had a rupture recently as I wanted to talk about my friends (including that friend) in MY therapy as it IS related to my issues but my therapist plain doesn't want me to talk about that friend. Probably because I was telling her a lot about that friend. All related to my issues, but I think my therapist felt uncomfortable. Though she would stress she compartmentalises different clients well, my friend kept worrying about The Major Thing since, I guess, my therapist isn't that great at pretending.