Before this, due to her parents' divorce and the involvement of a mistress, it was Cai Yiren's mother, who was also a friend and neighbour for more than a decade. So, after things happened, Lu Weixi instantly cut ties with Cai Yiren. In school, they verbally clashed every time they met; suddenly, tensions ran high.
Qin Zhigu should have originally sided with Lu Weixi but because of something that happened on Qixi festival, he also harbored some resentment towards Lu Weixi. Moreover, Cai Yiren was performing well and even always "stood by" Lu Weixi's side during their conflicts, like a hardcore fan of their couple.
So although Qin Zhigu verbally condemned mistresses when Cai Yiren was cursed, deep inside, his balance was subtly tipping towards her side.