If he is over-tightening them then they're probably too big/wide for him. Snowboard boots are not slippers they should feel a little stiff and a little snug in the store as they will pack out and soften a little with use.
It could be a habit too... I used to buy boots that were too big and got used to over tightening to compensate. When I finally bought proper fitting boots it took me a while to realize I didn't need to tighten as much. Overtightening a tight boot hurts just as much as overtightening a loose one!