Someone needs to be the devil's advocate here. The "gunsmith" was brought a rifle with the lock/flash hole,/breechplug already installed. As such, he did not have the option of moving the flash hole further away from the breechplug. His failure to drill the correct size for the 1/4 inch white lightnin liner was his fault, or is this what happened. Perhaps he did drill the right size, but the drill cut into the larger clearance hole for the breechplug, and therefore did not have enough threads to hold the flash hole liner. Drilling for a larger liner certainly did not help the situation. I suppose one could have left the breechplug in place, and originally drilled the barrel and breechplug at the same time, tapped them both and installed the flash hole liner with no issue, except that the breechplug could no longer be removed as it would be locked in place by the flash hole liner. It seems to me that the gunsmith, if he indeed is one, is not familiar with traditional muzzle loaders. Many folks declaring themselves to be gunsmiths today are actually nothing but parts swappers. Some of the fault here lies with the gun owner for taking his rifle to an unqualified craftsman and asking him to do something that was problematical in the first place.
Ideally, the "gunsmith" should have thought this situation out and explained the problems to the gun owner. If he still wanted to proceed, then the barrel and breechplug should have been drilled and tapped for the flash hole liner together, and liner installed. All should have then worked, but as previously noted, the breechplug would no longer have been easily removable. The post that just came in by Dpharis has an answer similar, but less verbose than mine. Check his post out.