There is no manufacturer for replacement barrels.
Unless he wants to put in far more money than it'll be worth- if you can find a smith willing to take on the task- just buy another rifle.
If I had a lathe and the knowledge, I'd get a .308 blank and fit it to the action. This would require no other modifications, using the same 54R brass and just size them for .308 bullets. A lot of factory ammo uses .308 bullets, anyway.