I think it's kind of you to be concerned.

T's can handle seeing people who know each other. Maybe at some point they will have joint sessions to work on issues. It would be unethical for a therapist to share any information with each client about the other, but unless you know that's happening, I don't see how there would be an issue.

The therapist could be completely competent to handle this situation or just be a terrible therapist and screw it all up, but I'm not sure how that would involve you, however. I'm assuming these people are adults and can make decisions for themselves about what works for them and their situation.