My own experience with ADHD was, unfortunately, not very positive. But that had everything to do with the parents, not the child. I was not informed of his diagnosis, until I put the pieces together and asked the parents outright. Mom and dad were in some serious denial, despite the fact that this boy was, indeed, on medication to help these issues.
She may not have mentioned diet, but it was the first thing that came up when I did a little research on my own. Some parents brush it off (such as the above mentioned ones, who sent thier kid with iced tea, Oreos, "fruit" snacks and other sugary crap), but in my opinion, it would be worth it to make the change even if it just has a little bit of an effect!
Find out from mom what the triggers tend to be for his behaviour, and what their coping mechanisms are. Does he need time on his own, does he need to be "talked down", etc. And hopefully mom/dad see the ADHD as an explanation, not as an excuse (for behavioural challenges).
One question though.... I thought the meds were given in the morning, essentially to get the child through the school day? I have not heard of afternoon meds?