This sounds hard, I'm sorry for what you're going through!

If I knew there was someone lurking around a few inches from T's door while I was trying to have a session it would completely ruin my sense of safety and privacy and my ability to focus.
I doubt that T would have much luck telling such a guy to back off, so although I would definitely tell her what's happening and how it affects me, I would also ask her to find me some other appointment time, when that other fellow wouldn't be there.