It's his wall, so he should build it on HIS side, unless an agreement has been reached.
Technically, that trench should have been dug in sections of 2 metres or so, That is to say, a 2m trench, then leave 2m undug, then 2m trench etc etc.
Alternate short runs. They should be concreted and the retaining wall built up. Then the "undug" sections are excavated and brought up....... joining them all together. That's just one way of dealing with a retaining situation.
His choice to build it, so his cost. Same goes for any new fence. Some people do try these things on. By jeopardising the stability of her drive, he's well out of order.