How do I obtain iPhoto for a second-hand Mac Air?
The only legal way to transfer the iLife applications from him to you would be if he had the original iLife installation DVD that used to ship with new Macs. Being that it's an Air, and doesn't have a DVD slot on it, it's unlikely he still has that media.

Unfortunately, you're stuck with re-buying the iLife applications from the App Store using your own Apple ID. Using his App Store ID to put the software on your Mac and then switching the ID to your account would definitely be on the wrong side of Apple's terms of service.