7 rem mag would be my choice for long range shooting. As others the 308 win will work quite handily and is and has been a sniping platform for quite sometime. I started hunting and shooting a 30.06 with a 165 grain btsp at 2680 fps with a Luepold 3x9-40 Vari X I. I hit 300 without an issue and have reached to 500 yards although can't say I consistently hit with it since it was a hunting rifle and never intended to be a long range platform. Good luck to your friend and hunt ethically if he happens to pursue game with it.
