I am probably the odd one out here, but wild animals left in the wild - stranded or lost - are still part of what makes the ecosystem work. If I came upon a lone wild baby animal, no matter how cute, it would still stay in that place when I left. In fact, I wouldn't even touch it. It is a part of life. It is how the animal kingdom works.
I guess if it were some obscure almost extinct creature, I might reconsider. But, that is just my opinion.
Due to the duck being in the OPs care already, they have to tend to it as it would be cruel not to.
However, I think we have scared the OP off.