I'd try Rich G's suggestion of seating and then pulling a ball to see if the patch integrity is being compromised in the absence of hot gasses. If it is, then options like Scotchbrite to polish the bore, or lead-lapping, or using valve grinding compound might work. Or, he could even have the barrel rebored and re-rifled maybe up to a .58 and be confident of starting life anew with a smooth barrel.
If the patch is getting shredded only on firing (i.e. the patch of the pulled ball tried in the previous step comes out intact), then experiment with different patch material, lubes, and ball diameters. If I was moving onto this phase of the experimentation, I'd try a few shots loading powder, topped with an equal volume measure of cornmeal, then the patched ball. The cornmeal will be a really effective firewall protecting the patch from combustion heat and gas blow-by, so if there is any damage to a patch, I'd attribute it to the patch being cut by the lands, or damaged by roughness/pitting in the bore. If the patches are intact, then start trying different powder charges, patch material (denim, light canvas, ticking, linen, etc), patch thicknesses, lubes and ball diameters until one emerges that works. I had a .40 that ate patches, and found that some patch materials would survive when lubed with water or spit, but not oil based lubes.
Good luck with it.