Can your neighbor cut a core into the dam and place it/compact it there? Tieing it into good soil past the ends of the dam.
If it was mine, and I couldn't re-core the dam, I would line the whole pond, carrying it up past the water line. My luck would be just doing part of it and have it seep around the clay in a year or 2. Then have to drain, remove all the fish that I bought and start from square 1 again.........
