There's a lot of advantages and disadvantages to both.
External definantly breathes more, you can usually attach more items to the outside and some like how it transfers more weight to the hips.
Internal keeps the weight much closer to the body, keeping you more balanced and allowing you to move more naturally. If you don't get an internal that fits well, it can ruin the experience

It shouldn't be able to move side to side and it shouldn't rub your back. I had one pack that did that and I returned it immediatly. Now adays they make internals that breath MUCH better so your back keeps cooler (which used to be a MAJOR disadvantage)
I have both but 99.9% of the time now I use an internal. I've taken both above 10,000 feet and both were capable. I just like the feel of an internal and how everything is inside the pack.
Just make sure he puts the pack on and puts WEIGHT in it! Thats the only way to find a good pack!