(SPOILER)Ben looked pretty shocked when Desmond said "You're looking for Faraday's mother, too?" I think there was just too much stuff going on at once for any of them to register much reaction. Also, it's worth noting that Ben and Desmond had never been depicted as speaking to each other onscreen before, so it's not like they ever had much interaction. As for Desmond not remembering the conversation, I just got the impression that it just didn't make any sense to him at the time and he didn't think about it until he had the dream about it.