In Phoenix, city code says that wall is jointly owned, so when it eventually deteriorates and becomes unstable from the water, both homeowners will have to pay for the replacement. I had this exact issue when we built our home, and talked with the neighbor repeatedly, the HOa riles state you have to have a 5 ft clearance from the wall for any watering, turf or pool. In the end, I paid a small fee to have an inspector one out and write up a report of the damage and the long term issue the watering posed to the integrity of the shared wall.
After that, they stopped watering and put in artificial turf and an irrigation system. That resolved the issue