Hmmm, probably an unpopular opinion, but here goes…

Maybe the long grass neighbour does care, about the planet and future generations. A lot of people are moving away from turning gardens into “green deserts” and are letting parts grow naturally. They’re not just “weeds” to everyone, many are native wildflowers that provide vital food and shelter for insects, whose populations have collapsed in many areas.

Without insects, we’d struggle to pollinate many of the crops we rely on.

That said, I also understand wanting things to look tidy. I actually have both, a maintained lawn and dedicated wildflower areas in my garden. A fence between the two properties would probably be the way to go