You will need to monitor them closely for signs of infection/illness. At the first sign of anything awry you will need to put the sick birds in quarantine.
Cold, miserable, wet weather is not something that should bother a duck too much. In fact, they seem to be the only ones that actually enjoy cold, wet, miserable days. They are pretty tough little critters. Your bird did not get sick because of the weather. It got sick because it was exposed to an illness (or had an injury that mimicked a respiratory illness).
I hope it was just a fluke and the rest of the flock remains healthy. Good luck.