If he were shooting at 400 yards or less I'd say any of the medium to higher powered .224 calibers would be great, but past there that little bullet is loosing a lot of steam and both drop and wind drift are going to be a big issue. I'd say use a .224 cal only is he's an awesome wind doper. Based on a quick calculation I just did, at 600 yards a 55 grain .224cal bullet at 3800fps at the muzzle will drift 41" in a 10mph full value crosswind. But if his call is off by 2mph the bullet will miss by about 8".

Based on his specs I'd go with a fast 7mm, or maybe at the least a 6mm/.243 or .25cal with heavier bullets.

But if I personally were buying the rifle I'd probably choose to go with a heavy barreled .308Win. It'll kick no more than a fast 7mm and it well known for and deserves its reputation for incredible accuracy.

Mike