I would agree with your friend.
I can't quote any specifics but generally anytime you open a package (in this case the barley grain) the item will spoil faster. Increased contact with oxygen and ambient moisture will dull the flavor.
The time frame that this happens depends a lot on storage conditions. If you are milling 50lbs of grain and putting it back in the original bag it will degrade fairly quickly. If you put the milled grain in sealed bags then it will last longer. Refrigeration will make a difference too.
I have my own mill now but I used to get my grain crushed at my LHBS. If I kept it sealed it seemed to last 3 or so weeks without much issue. Basically, if you can taste a difference it matters. If you can't, it doesn't.
Not really very helpful, I know, but there is no definitive answer available. To me at least.