Legal Question in Real Estate Law in California

It's difficult to give you a useful answer without more information, including (1) is there a written agreement somewhere governing this sharing, such as an easement agreement? (2) Are the well water being used primarily for agriculture or for domestic purposes? (3) How low is the pressure when lowered? (4) How is it lowered, i.e., does he have to go from his house to the well to flip a switch, or does the pressure drop because he draws heavily on the well, or does the pressure drop result otherwise? (4) Have you considered storing water in a tank on your property and re-pumping it from storage for your use? (I once owned a farm where my domestic water came from a shared well, but it first went into a storage tank and then we pumped from the storage for use inside the house.)