Exact same scenario, had this happen, may be of some help to you.
POCO had loose (barely functional) neutral splice at pole, pole grounded properly - meaning a ground wire down to the earth.
Service grounded properly with a ground rod and water pipe ground.
Neutral path had such high resistance, the over current had established a path through the earth for a percentage of current.
POCO non-responsive, saying "It's still there" and would not remake splice, even though it was quite evident it was rusty, nasty, loose.
Climbed pole myself, cleaned and crimped new splice, problem solved.
IOW, check your neutral connections ALL the way back, I'm willing to be it is what you will find. If he has close neighbors, you can also be having a problem from one of them.
I'm reminded here of one Creighton wrote up where a solenoid from a neighbors washing machine was causing significant problems in the house because they were both tied to the water line, in my case the house was very rural so neighbors did not fit into the picture.
Let us know, this sounds interesting.
