His main (and only) issue with the iPhone seems to be battery life. Interestingly, tests have shown that the iPhone's battery life is about on par (somewhat better, in fact, see links at the end) than other 3G phones' battery lives. However, people are using the iPhone way more than other phones, so the limited battery life is more noticeable.
So he basically returned the iPhone because he liked it so much that he used it way more than any other phone he's owned.
Simple solution: Turn off 3G when you don't need it.
The other issue present on the iPhone and not on most other smartphones is the lack of a removable battery. I suspect battery packs and power-related accessories are going to be pretty popular with the 3G iPhone.
So he basically returned the iPhone because he liked it so much that he used it way more than any other phone he's owned.
Simple solution: Turn off 3G when you don't need it.
iPhone 3G battery life comparisons:
http://www.anandtech.com/gadgets/showdoc.aspx?i=3353
http://gizmodo.com/5024804/iphone-3g-battery-life-beats-the-...