As he continues to stare, the glass of the window is suddenly pulled back somehow and transformed into a small cube. What, might he say, the fuck???

He doesn't even have to pretend to be surprised to see Wei Wuxian, accompanied by Klaudia. Of course. If Lan Wangji is here, naturally Wei Wuxian would be with him if at all possible. He's not entirely sure how to greet him though, if at all, so in order to give himself more time to consider his options, Nie Huaisang is just going to let his eyes roll back and his knees buckle and he's going to fall into what looks like a very convincing faint.

This way, the greeting will be up to Wei Wuxian, and it will be much better to judge his position from that.