When Fiorella's fourth grade teacher talked to the class about small thefts that had occurred in the classroom, Fio went home that afternoon and checked her bureau drawers to be sure she wasn't an unknowing kleptomaniac. Yes, your Fio has an overly strong sense of guilt.
But some people have no sense of guilt at all, no sense of right and wrong, no conscience. It's marvelously freeing to be around a person like that, to be able to express whatever snarkiness comes to mind and laugh about it, to be coaxed out of post-utterance guilt. But ultimately, there's a price to pay--a lack of conscience means the person has no sense of responsibility to anyone, including you.
Fio has been wrestling with the dregs of this situation for months. The only way to bury it is to understand it.
But it still makes Fiorella sad.