So sad to read of your loss. Thanks for joining the forum.
I don't think ducks get over losing a friend though they can come through to the other side of grief (I have seen it in our flock), and other duck company is very important to them.
If you can spend extra time with the lone duck, that would help, and yes I would start right away looking for company. It takes time to introduce them, but most ducks get along pretty well.
There is a rehoming forum on BYC, that might be a place to look.
If it was a predator attack, I hope you have been able to see how the predator got access and have fixed that so the remaining duck is safe. Whatever the reason, I hope it's been taken care of.