Outside of the appointment, I would not go up to a therapist or want to meet their family. I would not want them to meet my partner/family either. But I probably would not go up to a client, student, co-worker or acquaintance either - although unlike with a therapist, I would probably say hello or wave if our eyes met or something.
But if the therapist has indicated it is okay and the client wants to, I don't see it as a problem for them.