If you know that the barrel was just emptied a week ago, thats a very fresh barrel. I've used 3 different small barrels, and suggest your friend just keep it sealed up and cool until his beer is ready to go in. I wouldn't put anything in it...water, sanitizer, whiskey, nothing. Also why they would wait 3 weeks after brewing to fill the barrel? I'd just wait until the most active fermentation is over one week or so, and fill that puppy up.