If he can get a bootable installer made, then he's good to go. The problem is that the package that Apple provides as a download must be launched first from within OS X to make the actual installer app that DiskMaker uses, and that package has to be run under Snow Leopard or later. At least, I assume it does. Maybe the package will run under Lion and make the installer, but the installer is what won't run. If that's the case, it won't matter because he can make a bootable USB installer using DiskMaker.
ALTHOUGH! Hold the phone here! I just inspected the El Capitan package from Apple using the app Suspicious Package and extracted the installer app right out of it! I can't fully test it, but it appears to be valid. Link below. They have a version compatible with 10.5!
Suspicious Package | Frequently Asked Questions
So here's what I would do.
- Download El Capitan direct from Apple as directed. Download DiskMaker X 6. Download Suspicious Package 1.2
- Open the El Capitan DMG file that was downloaded. Open InstallMacOSX.pkg that is contained in the DMG with Suspicious Package.
- Using Suspicious Package, while examining the pkg, click on the "All Files" tab and look for the content Install OS X El Capitan.app. Right-click on that, and select "Export to..."
- Now use DiskMaker X to create a USB installer using that app you just exported.
This has a good chance of working, so long as what DiskMaker is doing isn't relying on something built into the OS that is exclusively found in 10.6 or later.
FYI, Store link for Lion. If you HAVE to buy a disk, you may as well get this direct from Apple. Some resellers are flipping used copies, and some are copies that came with a new Mac and ONLY have the drivers for that specific Mac.
OS X Lion (10.7) - Apple