When the daughter got belligerant that would have been the fuel right there to insist to see her checkbook of who was paying the mortgage where she was at. THat would have caused Dad to come out and handle the situation...
Now when the slide went up in my yard I would have drug it over to their front door, rung the door bell and would have had a LONG discussion with my neighbor - and his daughter.
Also - if your son has been riding the 4-wheeler in the same place for a period of time (historically that is) and no fuss has been made I would have discussed that as well. One of two things; 1. - Either move her car, which she could EASILY do -or- 2. Being considerate, get a garden hose and wet down the area which is causing the dust to fly... not a mud hole - but enough to knock the dust down.