I don't know how big your work population is or the town you're in or what a "casual" friend is. You seem to know a lot about his problems (might be what your T says is his boundary crossing, he tells you more about himself than you want to know).

I'm curious how your T knew it was the guy you know and discussed it with you? I never gave people's names to my T anymore than I gave T's name to others?

I would decide what I wanted out of all this. If you want to learn to keep better boundaries, I would leave things as they are and practice being clearer with him during any discussions so he does not give you more information about himself than you want to hear and knows what sorts of questions about yourself you are not interested in answering. I would also ask questions of T and make sure I understood the whole transaction they had that your T thought it was a good idea to share with you the information that he had contacted her and was seeing her and what she said in return about you/how she knew he knew you (If I told my T I knew "Xtree" I wouldn't like her telling Xtree about me! He may have just picked a T and happened to have gotten yours to go see but I would want to be sure of my T and that the T was not a potential source of difficulty for my own interests).