I'd think cocci too, given that it's affecting both ducks and hens, so won't be Mareks or DHV. Do the hens have the runs? Hard to tell with ducks!
You could just treat them for it. I know, before anyone says, you shouldn't just treat but what is she to do, wait for more to die?
The effective treatment for coccidiosis you have to get from the vet. Baycox or Intradine. Don't bother with Coxoid, which you can buy online, it doesn't kill the bugs, just stops them breeding, which is not enough when you've got an outbreak.