Get a FiiO E11 HP Amp along with a Line-Out-Dock (LOD) cable like FiiO L3/L9. You will get answers. The cable will bypass built-in HP amp,using just Apple's DAC and signal fed to fiio amp. The amp has all the bells and whistles that you need like gain,eq,replaceable battery. The total price would be <$80-$90 max.
NAh, he can't. Iphone 5's lightning connector is only digital, so you can't use fiio LODs as it doesn't have line-out. So unless he goes for an iphone compatible DAC/amp combo, most of which are in the $500 range he can only double amp through the headphone-out or go for a bulky lightning to 30pin adapter connected to a LOD but then the crappy DAC in the lightning adapter is used and not the one in iphone 5.