I think the situation QM mentioned would be somewhat bothersome. If I needed to talk about people including my friends in therapy, I would not like to have restrictions who can I mention or talk about in therapy. But the other way around - I would not want my friend to talk about me to our shared therapist. So I think it is fair enough request going both ways and it would be ideal not to share same T. But if that friend is not an essential character in the things that brings one to therapy I think it is doable and not a major problem.