I have a video linked in my signature from our trip last August. I used a handheld mount and found that to offer the best perspective, though I think preference for head strap vs chesty mount vs handheld is really a matter of personal taste. Personally I find it easier to shoot smooth footage with the handheld. It also depends on whether you like the first person perspective (i.e. showing what your hands are doing) or not.
Even if you're not in the front seat of a ride, you'll get a great view. The angle is wide, something like 170 degrees. It gives you a great sense of the overall environment and not just the back of someone's head.
On the water slides you are allowed to use the camera but you can't have it mounted on anything. This is true in both Blizzard Beach and Typhoon Lagoon. I handheld the camera inside its waterproof case on the slides there. I also used the Floaty Backdoor, in case I dropped it, and secured it to my wrist with a tether. I don't think the Floaty Backdoor is entirely necessary though; it's not like you couldn't scoop it off the bottom of the pool if you needed to - not like being out on open water.
If he doesn't have a spare battery, make sure to get one of those. GoPros have notoriously short battery life. You can buy 2 Wasabi brand batteries plus a charger on ebay for about $25. They last just as long as the GoPro branded batteries and are a much better value.
