I am going to go out on a limb here and venture a guess your buddies dad got his rifle and a spare barrel from the Civilian Marksmanship Program.
At the same time he also bought a Mann Accuracy Barrel from them.
These were not actually made for the M1 Garand but were fitted up to Springfield and Enfield actions for use in testing the accuracy potential of incoming lots of Service ammunition.
The CMP was selling the barrels seperate at one time, now they offer them fitted up to Springfield actions.
Scroll to the bottom of this link and see if the barrel in this link looks like the barrel your buddies Dad owns.HTH