As others have suggested, I think that you really need to talk to your friend and ask that you no longer discuss your therapist with one another. If you feel that you need extra backing or an "excuse," you can say that your T has requested that you no longer discuss him/your sessions with one another (and tell your T that you need this).

About 2 years ago, I made the HUGE mistake of referring a friend of mine to my T, who I had already been seeing for 2 years. At first, it was fun. We could discuss T, compare notes, and share this common experience. Then, it got ugly. My "friend" started making fun of me for having "mommy" feelings for my T (I don't have a mom), she told all of our mutual friends private things about my therapy, and she told everyone the name and information of my therapist. In addition to that, she simply became an abusive friend. Since I had been with my T for years and she was a new client, I told my T that I was no longer comfortable with the situation and I explained the abusive behavior and breach of confidentiality. My T's first response was that my friend and I should no longer talk about therapy with one another. So, I talked to my friend. My friend said "okay" but didn't actually follow through. So, I told my T again. Then, my T talked to my friend and asked her no longer to discuss therapy with me. My friend stopped for awhile-- and then started again. So, I talked to T one last time.My T said that i should no longer be friends with this girl, but that she felt she could still continue to see us both. I was upset-- I felt it was unfair-- and I almost stopped seeing my T. But, in the end, I cut the friend out of my life and stayed with my T. Unfortunately, I still see the girl around from time to time, and she still mentions my T when I do run into her. So, what I learned from my experience is that seeing the same T as a friend-- and discussing it-- can really be poison both for the friendship and the therapy relationship. If you want to keep both, I would strongly urge you to talk to both your friend and your T about putting a boundary in place.