I'm assuming that her ontology for anything but the most immediate, important, tangible things would be practically useless.

In which sense useless? She's a contemporary, educated girl, she can navigate this world perfectly well and you probably won't disagree with her about much in the descriptive sphere. What you would disagree about is the normative sphere, but that doesn't have to do much with ontology. Why do you assume that her "world-model" is botched? There are plenty of very bright religious people.