There was Yin in the well, but it wasn't dense. People wouldn't be affected if they didn't stay here for long. In addition, this place was far away from the crowd, so normally visitors wouldn't come here. Therefore, Gu Ning left it in the well and walked away.
On her way out, Gu Ning entered Dongfang Qi's sight, so she deliberately answered the phone and showed a resigned look. "Fine, see you another day." After that, she left.
She purposely did that to fool Dongfang Qi. Although Dongfang Qi couldn't hear what she said, he might read her lips, so she made him believe that her friend couldn't come and she walked outside afterwards.
Gu Ning was right. Dongfang Qi indeed couldn't hear what she said, but he made a guess from her lips and body language. He saw her walking out later, so he thought he had guessed correctly.