I live in a small enough area that this type of thing is pretty inevitable. What is your concern? That the therapist will share something with your friend? That you might see your friend in the waiting room? That you want to talk about your friend in therapy?

I can pretty much guarantee that there's no way that your therapist will tell your friend anything...even that you see him. Doing so would be a major breach of ethics and could easily cause him to lose his license. My guess is that he is pretty skilled at compartmentalizing multiple patients, so it probably wouldn't even occur to him.

For me it would be more awkward to explain to my friend why I didn't want him to see my therapist than to just go with it. But living in a small town, I'm quite used to quasi dual relationships and people showing up in places where I really wish they wouldn't.