What if Leng Shaoming couldn't accept it after Zhan Zhiyin told him the truth? He might stay away from Zhan Zhiyin afterwards and Zhan Zhiyin might blame Gu Ning for encouraging her to be honest.
If she told her to keep it a secret, it might seem as if she supported Zhan Zhiyin in fooling Leng Shaoming, which wasn't fair on him either.
Therefore, whether Gu Ning suggested Zhan Zhiyin be honest or not, it wouldn't have a good result. Therefore, it was up to Zhan Zhiyin.
Even if Zhan Zhiyin kept it a secret from Leng Shaoming, she might not be able to do that forever. Her friends might keep it a secret too, but other people wouldn't help her.
After all, there were other cultivators, monsters and ghosts in this world.
