Throw wildflower seeds (non invasive) over onto his perfectly manicured yard.
Another one is a long con: befriend crows, get them to come to your friend's house to feed. The neighbor will likely sho them away which will aggravate them. Crows hold grudges for a REALLY long time and only shit where they don't eat, aka his yard.
Always max out at 4/5 effort, so there's always room to grow.