Seems like a ride behind mower that is creating a path. Too heavy of a mower on moist soil?

Heavy mower + cutting Bermuda at 4 inches = this

Make sure he cuts in different directions each time (one cut parallel to the street, then the next cut perpendicular to the street). How often is he mowing and how tall is it right before he mows? Do you notice any thatch build up?

Needs to be cut lower. Did he cut it super short to start the season? Literally almost scalp it. Not sure what type of Bermuda, but should be cut at a minimum 2".

If you tell him TexAgs has conferred on this and offered up the solution, how could he get angry?

alternate the pattern N-S, E-W, NW-SE, SW-NE. Mix it up.

It doesn't have to be a heavy mower to cause these lines, and it doesn't have to be wet. Any normal push mower will do this if you continually mow in the same direction and the same lines. I had a neighbor who seemed to intentionally create these lines in his yard in a diagonal pattern that drove me crazy. As others have said he needs to change up his mowing routine to mow different directions. Shorter cuts would also help hide it. Ultimately, he could continue mowing in same direction as long as he moved the mower over some and didn't continue driving down same lines.