What do you mean she has adopted the duck? Just that she is watching out for it while it nests?
She doesn't really need to do anything for it other than, as she is doing, keep workers,pets, children and other curious eyes away.
If she wants to put some water out, that would be ok but that is all she needs to do.
Duck moms beef up in preparation of nesting; they then sit on the nest, getting off of it very rarely (she has to stay on the eggs to keep them warm) until they hatch, at which point she will get up and her whole family will move along, following behind the mom. They only eat grass and bugs in the wild, that is their natural diet. Leaving food out by her will only attract predators, and put her & the nest in danger.
Hope that helps!