I'll tell you why the iMac is OK; it's because the powermac sacrificed itself and absorbed the energy, thus being fried first.

Nothing in this world happens simultaneously. Something is always a few microseconds ahead of something else. Since the powermac actually absorbed the spike, there was not enough energy to fry the iMac.

Does that mean that the powermac was less robust than the iMac? Heck no. The powermac might have had shorter AC wires leading to the wall (first power strip instead of the third?) and this is actually received more energy than the iMac.

Next time, sell him a good surge protector.