We're having an open bar at our reception.  A good friend, let's call him Sammy, is a [functioning] alcoholic.  He went to his best friend's wedding a couple of weeks ago and when I asked how it was he just responded "I was f'n hammered all night, have no idea." 
I'm wondering if I can instruct the bartender to only serve him beer, no hard liquor.  If this is the case, I'd probably tell Sammy in advance.  I think he would take it well; we're very honest with each other.  
Is this rude?  Are there other ways to handle it?  
The open bar is not consumption-based; we're paying a flat fee, so it's unrelated to cost.