If there were no wires down in that entire mile from the primary down to his house, that primary down will be the biggest suspect culprit. Still, it is hard to imagine it causing the surge to extend to your customer's house unless there were secondaries run the whole way (which you say is not the case).
Any overvoltage on the primary would have affected both 120 phases and everything 240. If all the damage is on one 120 phase, that sounds to me more like an arc-over in the transformer, or a fault of the other phase to neutral somewhere.
I suggest a test. Find another surge protector exactly like the one that failed. Apply 240 volts to it and see if it fails. Some will and some won't. If it does not fail, then there had to be more than 240 volts.
A closer inspection of damage to appliances might give a better idea of what the voltage involved was. Any change it might have been an UNDERvoltage issue?
Did the homeowner report any lightning events? I have experienced lightning in snow and ice storms before (but luckily was not hit by it).
