Will she be suspicious of Lan Wangji remaining silent? Probably not. The man could go several days without exchanging a word with another human and people will probably just assume he is in a good mood.

But he acquiesces, and if they have to make small talk to keep from rousing suspicion, then so be it. Particularly in Wei Ying's case, who couldn't stop talking if Lan Wangji so much as answered him with "mn."

"You are a good teacher," he responds, saying so as if reminding him of something they didn't actually agree on but which doesn't seem so suspicious as to why Wei Ying isn't saying much. They've been playing. Lan Wangji has been learning flute.