I'm not a duck owner, purely chickens, but I've had plenty of broodies, and I can tell you, after more than one "cold hatch," they do absolutely fine as long as they are out of direct weather.
I've had broody hens go broody in the most wicked of weather (think my Pacific Northwest turning into freezing rain over snow highs of 20 to 30 F with overnights at 10 to 15F and blowing winds. And....I had a gals go broody during these cold snap in a coop nest with no light or heat (I burned a coop down so never add electricity to my coops now).
I've had numerous hens hatch in the colds of January and February, with phenomenal results. The babes hatch fine, and the mommas have them out scratching in the covered run by day 3 or 4. It's hysterical seeing the little babes running around in their down jackets using momma as an occasional warming hutch with the blustering snow all around. It totally goes against all modern thought that chicks need sterile brooders with heat lamps.
I will say that with my chickens, and I had a gal take off into the brambles, I do have to be extra alert to predators who want to raid the nest eggs or grab the baby chicks.
However, if momma is steadfast, has good cover (sounds like she has a cozy corner), can get to food and water easily and back again to the nest, and she is fairly protected from predators and those who can disturb, my volunteer bramble broodies do an amazing job.
But again that's my experiences with chickens. Ducks should, I think, be even more nature savvy.
Good luck on your duck momma
LofMc