Along with the other good suggestions, I would get her some niacin. You can add a little brewer's yeast to her feed, but check the poultry vitamins to see if niacin is in them. Ducks need more niacin than chickens as they grow and poor growth, bad balance and crooked legs are the main result of not enough niacin. Of course too much isn't good either. So, if you get poultry vitamins with niacin in them, don't add brewer's yeast.
A little bit of hot pepper (cayenne) is a good tonic for poultry. Also a bit of raw liver, if she will eat it, is highly nutritious. Only give a small bit at a time though, it's nasty if it sits around!
Good luck with your duck.