Your fiance would have to show that somone had reported the downed stop sign to the county, or that they otherwise had actual knowledge of it being down; and also that there'd been enough time to take action to repair/replace it (i.e. it didn't go down earlier that day or the day before). Otherwise, if they'd had no knowledge of the problem or no opportunity to fix it, they would not have done anything wrong, would not be at fault, and would not be liable.