There is an important clue in Bryan's second sentence.
Bryan: Okay, I always need to think about that person on the picture, who I thought was your friend. But they are not the same then.
Cathy (looks closer): No they aren't.
English uses the word 'then', either at the beginning or the end of a statement, to indicate that the speaker is seeking confirmation that the statement is true. In speech, a rising (questioning) intonation may be used, and in writing or print usually a question mark is used.
A 'No' reply confirms a negative statement, and a 'Yes' reply confirms a positive one.
Bryan: They are not the same person then? (Negative statement query)
Cathy: No they aren't. (Reply confirming that statement)
Mother: The car won't start.
Son: So we aren't going to the beach then?
Mother: No, we aren't.
Father: The car is fixed!
Son: Then we can go to the lake? (positive statement query)
Father Yes, we can. (Reply confirming that statement)