Quote: I know that there's a diffraction problem on the K20D (and other DSLRs) when closing over f/11
This will happen with all cameras, diffraction is a property of the lens, not the sensor.
What might happen is that his lenses are not telecentric. The sensor on a DSLR has more trouble accepting light arriving at strong angles, it's better at collecting light hitting it perpenticular to its surface. Film is better for this.
A lens designed for digital will take this into account, and be telecentric, which will help, indeed. But it should not have that much of an impact on resolution.
I suspect, since your father is using manual focus lenses, that he's simply having trouble focusing his lenses on a digital body. The viewfinder is much smaller, it's harder to tell if the subject is in focus.
A couple of solutions could be:
1-purchase a focus screen
2-use live view to focus
3-Get an AF lens.
I hope this helps!