It sounds like the nasal spray has a molecular structure based on the shape of a key molecule unique to Covid. Is it possible that your housemate thought they were experiencing regular hayfever but actually it was hayfever + fighting off a mild Covid infection at the same time? That's the only way I can imagine to explain what you described.
It could be that any nasal spray would have had that affect, even just a weak water only one. Considering that nasal irrigation is a treatment for hay fever, I can see it having some effect even if the medicine part isn’t applicable.
