water runoff problem with my neighbor?

I live on a small hill and the house has an elevated drive way on the left side of my house. Recently my neighbour on the other side of the drive did some landscaping work for his back yard.

The problem began: he has dug the ground about 7 or 8 inches lower, and built a wall overlaying rocks to have his back yard two layers. , it is kind like a h shape, the long side of h being the drive way wall (brick), and the rock wall touches the drive wall like the shorter curvy line of h.

since he dug the ground lower, the base bricks and some dirt underneath my driveway is now exposed, and he placed the outlet of the water runoff system right at where his rock wall and my drive way touched. During the heavy rain a month ago, some dirt have been washed away under my driveway, and the section of my driveway sank a tiny bit.

How should I go about this with my neighbor? although our relationship is not the best, I still want to have a positive relationship with the person...