My friend has a son with Asperger's, the kid has the worst time at school. Other children bully him and the teachers are largely uneducated about Aspergers. The interesting thing is that when this kid gets out in nature he is brilliant. He makes a better hiking partner than most adults I know. Why do you think he is so much better when in nature?
We have a couple of scouts with Asperger's and have noticed the same thing. My oldest has Sensory Processing Disorder (on the same spectrum as Asperger's and Autism) and he loves to be outside too. In general, these kids like the preprioceptive input. In English, this means the motions and impact of the joints while walking/moving provides them position feedback. School is hard for them because their bodies feel strange when they are not moving and most schools have lots of sitting. It is OK for them to enjoy moving while outside so they don't have that stress of "am I going to get in trouble for this".
Also, hiking gives them some off time from all the sensory input around us. We are used to the noise everywhere but for some kids it is really hard to filter out one noise to give attention to another. For example, if the air conditioner is clicking, my son will have hard time remembering anything the teacher says. He will remember the click, click, click, click of the air conditioner. What is really neat is how they experience the hike different than you or I. After a long hike I can ask my son what was his favorite part and he will usually tell me something I didn't expect like "the wind on my skin".
As with any challenge, some kids are different than others. For some of these kids being outside is hard since it is "open" and they like more confined spaces.I have found a short 2 mile walk is like a reset button for my son. It's great that your friend has found a place where his boy can feel at ease.
