If he is definitely going to fertilize than I would just keep your chickens in for a week or so to give it time to seep into the ground. However, see ifyou can talk to him at all. There is still time to return the fertilizer and choose something else!
There are natural fertilizers. Manure is wonderful and does just the same if not a better job of fertilizing when spread correctly.
Anything with chemicals in it would be a risk to animals, even dogs and cats. Also, if your chicken coop is at the bottom of a hill and you fertilize any ground...