Actually, it was implied from her "group therapy" sessions that she was messing with his head for quite some time. She was pushing him to talk about painful and embarrassing experiences without offering any sort of warmth and or support. In fact, far from actually helping any of the patients, those groups seemed just an excuse to belittle and/or rile them up. The calm, cold way she sits and watches that fight break out in the session speaks volumes. So, no, that third act didn't exactly come out of nowhere.