"My friend will not want to tinker with the OC in any way once I set it up. There is a reason he asked me to build it for him."
Go for the i5-8400 - less hassle yet more performance in games. Easy decision.
Or save him a fair chunk of money and set the 1600 to 3.8Ghz with some cheap RAM at 2933Mhz. Put the rest to the RX580 and he'll have a system that will see him past at least one monitor upgrade. Maybe two with DX12 and Freesync.