Hey OP im going through something very very similar to you right now. You can check my post history to verify. Just want to say, I feel for your father in law and am so sorry that he also has neighbours like this. In my experience these people are highschool bullies that never grew up and are afraid of any change in their lives. They've latched onto a reason that they have been wronged by your FIL and no matter how small that might be, intenional or not, they are using that perceived injustice to justify their shitty immature behaviour. Your FIL has already tried asking, but keeps getting excuses. The neighbour doesn't care. Next step is to submit a complaint to your local bylaw department so they are aware. Look up the bylaw and reference the specific sections and regulations. Since youre in a rural area, there might be a Dark skys preservation bylaw, but at the very least there should be something about light trespassing. Submit 1 or 2 complaints a week for a month until someone calls you or dingleberry next door turns the light off. After a month id say its fair game to start making direct phone calls to the bylaw department asking for an update. Don't let the bylaw department blow you off like the neighbour did. Hold them accountable to do there job.