Musheng, Xiaosong, and Zhao Sishang took a car to the workplace of Rachel, where she was working. During the ride, Xiaosong casually asked about Zhao Sishang's whereabouts just now. Musheng remained expressionless, hearing Zhao Sishang's calm reply, "No, I just went to chat with a friend."
This was a very normal response. Had Musheng not heard otherwise, perhaps he too would have chosen to believe Zhao Sishang.
It couldn't really be considered lying because, judging from Zhao Sishang's attitude during the conversation, the person with whom she was speaking could indeed be considered a friend. The only peculiarity was the content of their conversation.
But she didn't bring that up.
