There's no money to be made in iOS-only development due to small ~10% marketshare and jailbreakers. He'll need to supplement with cross-platform Android development like most developers have discovered.
For example, lets compare paid version of the most popular media player on both iOS and Android.
Android
MX Player Pro $5.99
500,000 to 1,000,000 unique downloads ($2,995,000 to $5,990,000)
68,919 reviews
Since Apple hides the # of downloads we have to extrapolate from the # of reviews which is about 1% so approximately $44,950 to $89,900.
iOS
nPlayer Plus $8.99
Apple hides the # of downloads
676 reviews