fouled plugs

that's looks like oil fouling to me. you might have oil coming down past the stem seal. see if he put the proper seals on. the seal should be tight on the guides and not the umbrella seals on the intake, the exhaust doesn't matter. the proper seals will have a noticeable edge on the top and the umbrella seals are rounded at the top. the umbrella seals do no fit fight to the guide and are only used on the exhaust.
another thing is oil maybe coming down pass the guide where it was pressed into the head after drilling for it. if the bit gets too hot the press fit will be wrong and not as tight as it needs to be.