I'd sell the Canon 24-105mm f/4
And then buy these:
Canon 8-15mm f/4L
Canon 16-35mm f/2.8L II
Canon 50mm f/1.8 (light, small, cheap, and great IQ. It also covers the gap from 35mm to 70mm and gives you a very low light option too that's not too long.)
Canon 70-200mm f/4L IS (the 70-200mm f/2.8L MkII is really nice but it's heavy and expensive. The f/4L IS is very close in IQ to the MkII and is much lighter, cheaper, and easier to lug around. I'd spend my real cash on wider & faster lenses for landscapes, vistas, on-board shoots, and walk-arounds. I'd bet the 16-35mm f/2.8L II would be on your 5D3 70%+ of the time. Do you really want $2k worth of lens sitting in your bag 60-70% of the time? )
This will cover virtually every shooting situation you come across.
Nah, he should rent them
Rent them for two years ?!?