I agree I would not feel good about giving this duck back. She truly does need a friend, even a stuffed animal would be a help and a mirror with safe edges.Seeing pictures of this duck breaks my heart. I'm glad that you're trying to help her.
Are you able to bring her to a vet? I know that it's difficult to find a vet that knows a lot about waterfowl and that it might cost more than you want to spend, but this poor duck needs more help than what most people on this forum can offer as advice.
I think my ducks would be terribly lonely as only ducks. I was given my first duck as an only duck. I got him a mate as soon as I could. His personality changed drastically -- he wasn't lonely anymore and was finally truly happy.
Since you seem like a person who really cares about animals, you might ask your friend if you can keep this duck. I'd hate to even think that she could go back to living in such an unhealthy state.
I wish you all the best,
Amanda