You must remember the reason for the C&R is collecting. Museums are professional collectors and every museum in the world buys, sells and trades to get what they want for their collections. It's an inherent part of the hobby - just read the Trader forum.
It's correct that you can't buy specifically for someone.
However there are no limits on how many or often you can sell any surplus or culls from your collection.
I've regularly bought in quantity (over 100 via Century's 3fers, 4fers) , kept the best, and sold and traded off the rest to feed my addiction/hobby. I now have about 40 collectible guns at a net cost of zero.
So if your friend doesn't mind a cast-off, it's OK, but you just can't promise him anything.
I swear by Jupiter Optimus Maximus .... in the army of the consul Lucius Quinctius Cincinnatus and for 10 miles around it I will not steal anything worth more than a sestertius in any one day.