There are a lot of young aggressive lawyers out there--sue 'em. A good lawyer will cut through the red tape and find out exactly who controls drainage in the area.
After you install it, Mr. customer will feel better if someone else pays for his new drainage system. Or pond.
You can always build a raised berm flowerbed...say 6 feet high...so no water would flow down to your customer's property until the level was 6 feet higher upstream.
Last edited by RigglePLC; 10-05-2012 at 10:15 PM.