I am so sorry for your loss.
I do not own any ducks, but I do have chickens. One of them, a RIR, stayed inside for a few weeks after we noticed she was ill. (We believe she had cancer or another reproductive issue) she unfortunately passed in our arms.
Fortunately, none of the other chickens seemed to notice she was gone once she had passed, I think they were used to her absence after the weeks that had gone by without seeing her.
I hope your other duck is not too heartbroken, and hopefully won’t notice too much.
I’m so sorry for your loss.