Yes your female will be very lonely and very vulnerable if left alone outside with out some kind of secure pen and house. I'd consider getting her a few adult friends or find her a new safe place to live. Being female it shouldn't be to hard to find a home for her. Maybe try an unbreakable mirror for company until you decide what you want to do and spend some time with her to ease her loneliness.
She seems a lot better today, she has been chasing the dog and staying close to the houseI am sorry for your loss.
. You should definitely look into getting at least one more duck, as ducks are very social and can become depressed and anxious when they’re alone. And if you don’t want to get another duck then I would definitely consider rehoming her. Until you decide I would take Miss Lydia’s advice add give her an unbreakable mirror to help with loneliness and maybe also some sort of stuff animal to cuddle up with 
