I would not call it unprofessional. If I understood correctly, they already knew each other before and just became better friends. That is life and they have every right to be friends or even lovers. But it is totally up to the client, if it is a problem. I myself would like to be informed to make a decision how to deal with it. If I thought the T was good and ethical and kept my stuff confidential, I would probably be ok with it. If not, I'd find another T. I would probably want to hear it from my friend first, not my T.