I agree about finding an owner if there is one. I am still grieving and I mean grieving after having a parrot escape, 6 years later. If you love a pet, you love them and miss them forever.
Actually though, if you think there is a reason why the duck might fly off, I don't really see a problem clipping the wings as wings grow out, provided the clipping is only of the feathers and not the bone tip as is the practice of some people who don't want their poultry to ever fly.
About the "inhumaneness" though or the failure of the previous owner to clip, you can get busy, the wing is grown out more than you thought, etc...
And I don't know your predator situation where you live, but we plan to get muscovies with other poultry, and we probably WON'T clip our free range birds, locked up only at night, because of our large number of predators, i.e., coyotes, bobcats, hawks. We will use lg dogs too, but we feel the bird needs some means of quick reaction as well.
Connie