@DuckyDonna, is a vet an option? If not, and you feel an egg, you could try giving calcium gluconate or calcium citrate and let her soak in a tub of warm water.
Maybe that duck's reproductive tract tore and deposited the shelled egg into the abdominal cavity, but I promise everyone that eggs do not travel back up the reproductive tract and get deposited into the abdominal cavity.
I have treated many egg bound hens, and I have necropsied many internal layers and have never found a shelled egg in the abdominal cavity.
Good links that explain what internal laying is:
http://www.poultrydvm.com/condition/internal-layer
http://www.chickenvet.co.uk/health-and-common-diseases/egg-laying-issues/index.aspx
http://www.scoopfromthecoop.com/laying-issues-internal-layers-and-peritonitis/