If the door was still open, and hence the cockpit door was still open, I'd have gotten the Captain's attention and simply said "Captain, I think we have someone on the plane who might have had too much to drink".

Of course, if the FA (or worse, Captain) then went back to him to see what was up he would probably be on his "best" behavior and the ship would still sail anyway. And, now, I'd be the one he and his friends are glaring at for the whole trip....