Why are you surprised? Ads in the app = instant uninstall for heck of a lot of people.

He should wait to see how the free graph will develop after a few days/weeks because chances are that download numbers will fall pretty quickly to ~1000/day.

Why wait until he's flamed out then? The same amount of dev time will have be spent whether he adds ads on day 2 or day 20, why miss the bulk of his opportunity at the top of the charts?