She hatched a bunch of them, and then you got this one reject egg, is that right? How many ducklings does she have? She rejects this other one? If so, then you can raise it indoors. It's not that different from rearing chickens, etc. I've never had ducks myself. I know that without a mother, young ducklings shouldn't have a pool of water in which to swim because they don't have any waterproof oil. Ducklings with a mother get some from her. My page on raising chickens is at
http://www.fishpondinfo.com/chicken3.htm but of course, some things are different (type of food, etc.). I think that ducklings also like it a little less hot. For the first week, 90 degrees F under the heat lamp is fine. Be sure there's a cooler area as well.