Couldn't happen here in Nebraska. No new Irrigated acres in our area.  I know it doesn't seem like a big deal, but what might be next on this neighbors list? If you let them get by with this you may come home to a fence moved on to your property. I know I'd be considered a complainer but I wouldn't grant them access.  In the "Old West" the saying was: "Whiskey is for drinkin'. Water is for fightin'."   "Do before you ask" is not acceptable.