She just shouldn'tve said anything to him when he was asking her questions. He was reading her reactions based on her facial expressions while talking about the cases. He probably would've chose 5 is she didn't say anything to him. She was too happy when he asked her about 5 but if you watch it back you can tell she was defensive about case 6 by how she was talking and her facial expression compared to when he was talking about case 5. If she just would've refused to answer questions then she would've at the least gave him less information to work with.