To be clear, Pyrite (fool's gold) does have value (albeit rather small, compared to more precious minerals).
Moreover, it can contain gold (increasing its value), or be indicative of nearby precious metals, meaning that large deposits of pyrite should usually be given some note.
Given how ambigious "a piece" is, let's assume a reasonably moderate one ounce weight per piece. Googling, it looks as though typical exchange value (not market value, which may be higher) is 55 cents per ounce. I can't tell what a "canned rice cake" is, as nothing comes up for that, but a typical bag of rice cakes appears to go for between 2.50 and 3.50. Meaning that, yes, 5 pieces of pyrite WOULD potentially be able to be exchanged for rice cakes, despite what the "expert" stated.
Moreover (though this can't be held to criticism to the individual, at least), pyrite had value for pre-modern civilizations as a firestarter (which is also the origin of its name, deriving from the Greek word for fire, "pyr"). Meaning that, within the context of her Viscounty, it could actually be a rather useful resource. Moreover, it can be used as a source of sulfer or iron sulfide (beneficial for things like herbicides, pesticides, or dyes)- though for that, it'd likely be a lot easier and cheaper for our MC to just obtain processed materials directly from her own world.