I can't promise that you'll have much success, since incubating is a delicate process. Do you have a thermometer you can put int the bowl? The temperature needs to be around 100F. You also need to keep humidity around 50% (I think thats right for ducks). Are you turning the egg at least once a day?
If he found an egg, I'm guessing the others hatched and the mother abandoned this egg. It may not be viable. You best bet is to post on the DUCK threads at "Other Backyard Poultry" forum- other duck folks would know how best to handle your situation.