The best alternative for your friend in that case would be to do nothing.

Krieghoff barrels are made from an alloy that is very near stainless (Bohler Antinit or modern production similar alloy).

The barrels are highly rust resistant but difficult to blue, and difficult to keep bluing in good condition with heavy use.

I'd advise him to just leave the barrel set alone, use as is, and accept the worn finish as a badge of honor.