I guess it's ok. I saw my T's husband twice. In the first time, I knew it was him because I'd seen his picture on her social media. Awkward, I know, but that's what you get for cyberstalking. This time he stopped by to bring her something she'd forgotten.
The second time he was there to fix something with the internet, and, by the end of my session, he was done and was sitting at the waiting room waiting for her. I felt it would be rude not to acknowledge his presence as I was saying goodbye to her (she was sitting beside him at the couch then) and I said goodbye to both T and husband.