I feel like she was able to react because she has an Enhanced sense of Sight. Like how Tanjirou has Enhanced sense of smell, Zenitsu Enhanced sense of hearing and Inosuke with his enhanced sense of touch. I'm gonna go out on a limb and say she has an enhanced sense as well because of the way she talked about the detail of the way his face is. Also while we're at it i'm gonna say that Genya has an enhanced sense of taste and that can explain how he's able to get demon powers by consuming them.