30-06 is one of those old calibres where they got it right right bsk in 1906. It is good for just about any hunting situation you find yourself in around the world. Lighter bullets at full throttle loads you have a flat shooting mountain rifle, a very versatile all rounder with 150 to 165gn bullets and 180 to 220 gr bullets you can handle really big game.
Only slight downside is that it has enough recoil that it necessitates good rifle fit and technique - but those are pre requisites of any shooting.
If he already has a 222, 30-06 would be perfect partner. And you can find ammo everywhere around the world.
The other calibres in the same vein are the 7x57 and 7x64 - the later is very popular on the continent as military calibres where prohibited post WW2 for a number of years. Does everything the 30-06 does but with a little less recoil. But ammo can be hard to find.