I could mechanize this with two brass (or even plastic) rods, two weighted floats and two plastic tally-counters.
Hang the rods on the inside of the tank.
Calculate the pressure required to depress the button of a tally-counter.
Set one tally-counter at the bottom of the tank at the base of a rod.
Put the other tally-counter on the other rod, with the button at the level where the float valve is set to stop refilling the tank.
Set the floats on the rods and weight them an ounce or two heavier than minimally needed to allow gravity to apply the force required to push a tally-counter button.
Calculate the volume of water required to raise the float on the float valve from the start filling to the stop filling position. Let’s call that a fill volume.
When water volume is reduced in the tank, gravity will cause the floats to descend along their respective rods.
When a weighted float descends far enough, it will increment the tally on the counter. The lower counter is only counting flushes; the upper one will count all volume drops.
As the tank is filled back up, the float will rise, letting the button reset to count the next tally.
Set the equipment in place for…oh, I dunno…Friday through Monday would capture weekday and weekend traffic; Midnight Sunday through 11:59 Saturday would probably be more accurate.
Volume drop # minus flush # equals number of leaks.
Fill volume multiplied by leaks equals volume of water lost during the time period. Multiply somehow to get a month’s water wastage
Multiply water wastage by water rate to figure out how much money is going down the drain each month.
An easier, time-tested method:
If a leaky toilet is dripping onto the floor it would be easy to notice and fix.
If it’s leaking into the lower section of the toilet, the extra water will eventuallly just overflow down the toilet-drain.
At closing time, drop blue or red food coloring into the tank(s). [Don’t use yellow, for obvious reasons.]
Before opening for business, check the bowl(s) – and flush so you don’t freak-out the patrons.
If water leaks from the tank to the bowl, the food coloring will change the color of the water in the bowl.
It doesn’t matter how much water is being wasted; any tank that’s leaking should be repaired.
BTW: If your friend is in a food industry, food coloring can be considered a business expense (i.e. a non-taxable purchase).