Honami has absolutely walked in on Kanamafu in some... compromising situations. Sure, she finds out about Mafuyu's true self (or at least, a fraction of it), and maybe that starts to shake the image in her head of the 'perfect honor student Asahina-san,' but I don't think that makes it any less jarring to walk into her job only to catch Mafuyu in Kanade's lap getting all up close and personal.
I mean, Honami has seen Kanade at her most pathetic, I can't imagine her expecting Kanade to have that level of game. Mafuyu is beautiful, popular, smart, and athletic, the kind of girl who could have whoever she wants, and here she is, absolutely smitten with this tiny little composer who can't even take care of herself. I have to imagine that comes as a bit of a shock (even if Honami is professional enough to probably not let it show).