You could meter the power source of the pump..
I share a well with another land owner. I would like to have an effective way to gage water usage in order to pay the electric bill to pump the water
** Not a professional - only a DIYr **
Install water meters or at least one.
The well owner should sell the other person water for about $250-300 a year and that should cover power and depreciation.
Anytime we have ran across this type of thing. Both houses would have a water meter installed.
For every gallon used it cost say, 0.25 a gallon. That money would go into a fund to repair the well when it would need service and to pay the electric bill for the well.
If the owner tried to charge a set amount, the other neighbor would use a lot more water than normal, because if he's being charged for it, he is going to use all he could.
After some time, the friends and neighbors were no longer friends.
"I shall never surrender or retreat" -Col. William Travis