@Riger i think he already tell her that is just illusion, yet she still think it real, he say go for it so she can experience how fighting against illusion mages...
@Riger Well, its badly made in the Manga i would say, originally it is that he tells her its a illusion and let her do her thing and afterwards explains that her way of doing it was wrong, since its useless to directly fight illusions, instead she should have tried to find the sorcerer that casts the illusion.
Its more a learn by experience thing, instead of a school-approach with telling everything beforehand, if its without knowing that it feels a bit weird tho, yeh