My vote would be against running an automatic float system.
Some variance in the water level of a pond is certainly more natural compared to a pond that stays at an exact level all of the time. As the water level goes down a bit, the shore plants progress down the slope a little bit, and then there is additional cover for all of the base of the pond food chain when the water level goes back up.
However, I think a failure in the float switch is a much greater concern. If the switch fails in the "on" position it could draw the well down and allow for silt intrusion into the wellbore, which can ruin the pump or even result in the loss of the well. The float switch is not going to be located in a clean water tank - it is going to be in a pond with floating leaves, algae, turtles, etc.
Personally, I would just put the well switch in a convenient place for the homeowner to access. If he drills a low-capacity water well, then maybe put a timer on the switch, so it will run for 8 hours (for example) to top off the pond. Further, if they are hosting an outdoor party, they could fill the pond an extra 6" to create an even "cleaner" look.
Of course, the guy paying the bills is "always right", so if he wants a float switch you should install the switch.
