Part of the diagnosis criteria for ADHD *is* that it happens in more then 1 setting.
Just because its close to home for me & my kids. I would suggest your friends has a read of a blog called My Aspergers Child & looks up the entry on the cross over between ADHD & Aspergers.

My ten yr old has ADHD behaviours at school but not home. I refuse to medicate him based on that alone. I strongly believe he is not ADHD.
Mind you, your friend will soon find out if medicating him for ADHD is working as Ritalin in a stimulate. It works by stimulating the area's of the brain that are understimulated (impulse control namely) but if these area's of the brain are functioning at normal levels & you add a stimulate... well you get a hyped up kid I guess.