Though Bai Qingshi and Su Hu did say a lot of flowery words to win over Bai Feng when they were locked up in the cell, they didn't mean it. Nor did they feel guilty over the things that they had done to Bai Meiyue. The only thing that they were honest about was feeling regretful.
Of course, when they said that they were regretful, they meant that they were regretful over the fact that they were caught and failed in ruining Bai Meiyue.
Thus, even though they saw Bai Meiyue leaving the building, neither of them asked Bai Feng to stop and help Bai Meiyue. Instead, they hoped that Bai Meiyue would be bitten and killed by those monsters who were lurking outside for all the trouble that she had put them through.