The owner thinks the cause is probably a manufacturing defect.
I'd say very unlikely.
How old was the prop? When did it last have an overhaul/calendar inspection?
I'd say more likely caused by an un reported prop strike or corrosion.
I really hope they find the true cause.
Some on here will remember the Lycoming crankshaft issue. There was a crankshaft which broke near the prop flange in the UK. The cause was determined to be due to internal corrosion in the hollow part of the crankshaft. Some time later it was discovered that that aircraft had had a prop strike which was the real reason for the crankshaft breaking. However not before many crankshafts were unnecessarily replaced at great expense.