Zacchus didn't see her, it was only his son (a spirit) who saw her. When the MC was saying his friend is able to clean, Zacchus just accepted that the man who gave him a chance to find peace with his deceased son has an invisible partner who is adept and cleaning and organizing.
How is this even any of Leon's business? Kohaku isn't in the Battle Guild, so Leon's title as a guildmaster doesn't mean much. Of course, Kohaku will shit himself after hearing it, but I'm just saying that Leon doesn't hold any authority over our naive doormat.