In what way is the camshaft damaged??
How did he go about confirming the oil had filings in it? ie:- checked the filter or removed the sump?
Normally when a turbo begins to fail the prime culprit will always be lubrication problems...unless something is ingested into the vanes..But if the turbo isnt getting proper flow of lubrication to shaft/bearings then this can cause the turbo to run hot eventually destroying the bearings the vane shaft run on hence the metal shavings unless the shaft fails or siezes up....To what extent the camshaft has failed? id say that could be because the metal filings have caused scoring from grinding metal shavings around..and may not be just with camshaft..could be bottom end bearings too...in which case id say the engine's basically trash sadly...as the cost to totally rebuild that engine and clear out all those small filings would seriously out weigh the cost of putting in another engine or the value of the car even with a good running engine...........In my opinion going by what you have said.because you have had the Turbo replaced and the problem is still present..would point to another issue on the engine and not with the turbo maybe...having said that..because filings are found in the oil.which they could confirm further by dropping the sump to se whats in there..wear damage internally could already be done..I think it would be overall cheaper to replace the engine..and thats basically what you would need to ring around for locally as per quotes aswell as sourcing another engine...