Fatigue failure is a statistical thing, which is why there's so much redundancy (and added cost) in commercial aviation. Inspection is designed to catch cracks and defects of a certain minimum size, based on an assumed high probability that they won't grow to failure before the next inspection. No part is perfect, and there will be a few defects smaller than the size the inspection is designed to catch. Visual will miss anything below the surface, such as inclusions and microstructural defects and anything that is obstructed by other components. Commercial airliners have much more rigorous (and costly) inspections that not only have higher resolution than the human eye, but can "see" below the surface of a part. Commercial airliners also have much stricter metallurgical standards for all their components, again at higher cost than private, so the defects are smaller, and the cost goes up exponentially as the size of the defects allowed decreases. And still there are terrible surprises.
In your father's case, when the mechanic did the inspection of the part, there may have been nothing to see. The initial crack may have been hidden and it may have encountered an inclusion or internal pore that let it propagate to failure prematurely. Likely your dad's mechanic did a visual inspection (maybe with magnifying lenses and even a fluorescent dye), because he didn't have access to the ultrasonic and other non-destructive testing methods that are available at the heavy maintenance facilities. Best he could do still couldn't catch every possible defect.
If you had the part, you could do a failure analysis (SEM fractographs are way cool) and see the initial crack, the region of slow crack growth, and the region of rapid crack growth. You could deduce whether the crack would have been visible during the slow growth phase. With 2020 hindsight, you could figure out whether he missed something, and whether the part was defective (I've had defective engine parts that failed because they didn't receive the correct heat treatment, that also happens).
Best to be thankful everyone survived the failure of the part and, even if you spend the money for commercial components and inspection, accept that there's always some risk.