You chose to be honest with the girl. A brave choice, seeing how confused and frightened the girl is after facing the dementor.

"Magic...? Oh my God. I'm crazy. I'm going insane. This isn't happening, this isn't happening..." She hides her face in her hands and shakes her head vigorously. "This isn't real."

Oh my, what a situation! There needs to be something else you can do. But what could make the girl feel better? You must do something and quick, before the dementor comes back... Or something worse...

Now, focus!