Take your pick of the popular deer cartridges in PA and go for it. I would warn him away from anything with "Magnum" in the name, just because he does not need it.. 243 WIN will take an elk, but I would not recommend it. 308 WIN, 30-06, 270 WIN, 7mm-08, are just fine.
There are a couple of provisos:
1) Make sure he can handle the recoil
2) Don't expect to make 500 yard shots unless he's practiced them thoroughly back home.
In relation to the latter point, I was out one evening and saw a herd of doe on a hillside about 400 yards away. I knew my 30-06 load would not hit the deer if I aimed for the chest, but I just wanted to see what would happen if I shot at a deer that far away. I loosed a round at the nearest one, and watched. The round hit a good 8 feet below the deer's chest and skipped along the ground under their hooves, making them jump a bit. I tried again with similar results. On the third round, the deer decided to leave, and by that point I was blind from the muzzle flash. My point is that a deer at 400 yards is nothing like a deer at 200 yards which is nothing like an elk at 500 yards, and buying something with "Magnum" in the name will not change that all that much.