Model 12 barrels need to be fitted to the gun and are not interchangable in the same sense as modern Mossberg, Winchester or R4emington designs are now. Most probably, both of your friend's barrels were fitted to his shotgun, which is why it came with two. (no choke tubes back then and a Cutts or Polychoke is an abomination on a nice old M12)

I have changed barrels on a M12 but it was a long time ago. Probably, dfariswheel will be along to give you the real lowdown on how to do this, but in the meantime, I will do some searching and see what I can find.

Here is a really good factory schematic...

www.urban-armory.com/diagrams/win12.htm

And some pretty decent info with diagrams on disassembly of barrel from receiver...

www.nrapublications.org/tar/Winchester12.asp