Sounds like the neighbor observes that your pond won't hold water. A key question is when did the neighbor's problem start? If when the pond was constructed, maybe? So, it seems he is arguing that when your pond receives water and rises above the horrible leak, that it very soon starts flooding his yard. There is a very simple proof to his theory. Dye the water in your pond. They make fluorescent dyes just for this purpose. When your pond floods and begins going out the leak, the dye should very rapidly make its way to your neighbor's yard. Otherwise, his theory is false.

It isn't what we don't know that gives us trouble, it's what we know that ain't so - Will Rogers