Hello Alf it looks like you are in uncharted waters, i could not give you a technical response but having faced similar i would lay it firmly with the customer not just because he's supplied a different head but a used one. there could be differences in the head or gasket or any number of unforseen difficulties, make sure he understands that he pays regardless of the outcome. sorry to be neg but been bitten a few times with similar scenarios.
good luck
chris
ps i dont think these heads can be skimmed or pressure tested or is not recommended