A few ideas popped into my head, might consider trying the following:

I'd clean or replace the flame trap and all its associated hoses first before going after the knock sensor. It costs nothing to check them and clean as needed. Replace if necessary. Even if it doesn't help, it is wise idea and important maintenance for your neighbor's brick.

Besides changing plugs, did he also check or replace distributor rotor, cap and ignition wires?

Finally, did he try some good, concentrated fuel system cleaner like Chevron Techron or BGK 44 (name??) to clean the fuel system?

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90 244DL 263k :)