I don't think the issue is just the two clients talking to each other. I think the bigger issue is both clients, individually, talking about each other (even if it's inadvertent). It breaks the frame of therapy.

Think of it like this. I'm seeing T for my stuff. I'm reserved, I don't fully trust T yet, and I'm cautious about what I share. As a client, that's my right. I get to control what T knows about me, and when I disclose things.

If my best friend goes to the same T, she may inadvertently bring up some of my stuff. She might not even know that I see that T (but the T may be able to figure out she's talking about me - it sounds unlikely - but I've seen Ts on reddit that have eventually realized that someone a client is talking about is someone that they know.)

So, my friend talks about how hard it is to talk to me, or how I'm stressing her out, or how clingy I am. And, maybe as part of that discussion, she starts telling things about me - like that she understands why I am like that, because of some trauma I've gone through, or something with my crazy family.

If T realizes that she's talking about me, and if I haven't disclosed those things yet - that's a huge problem for me. T doesn't get to have outside info about me - that's part of the frame (for me, anyway) that helps create safety. I'm supposed to be in control of what, when, and how I disclose my stuff.

It might be that a good T can keep all the stuff separate, but since Ts are human, I wouldn't count on it. It's easy to forgot where you heard something, and if my T suddenly thew out something about me that I had never disclosed, I'd be incredibly freaked out and upset.

Plus, it's going both ways. T has to remember to do this with both clients.

I also think Ts are supposed to have a certain level of objectivity, and I think that would be hard to maintain if two clients know each other and are close enough that they're talking about each other in therapy. If I'm complaining about something hurtful that my best friend did, but my T understands the real reasons why that friend acted like that - how can my T support me, without being torn between the two clients? I want my T to be unequivocally on my side - not making excuses for someone else's behavior. (This has been an issue with me with past Ts who have a ton of compassion for my brother, who's an addict, and feel pulled to support him rather than look at what I need in the moment. Like - one actually came to the waiting room and handed me a book to look over, because he thought it would be good for my brother, and I should buy it and mail it to him in jail!)

So, even without talking to your friend about the T, it would still raise all kind of concerns for me. Lots of potential for things to go wonky. If there are no other choices, then I guess you just deal with it. But in a bigger city with lots of choices, I'd expect a T that already had an established relationship with me to turn down my best friend.