Just an off the wall thought...what about a 12 volt water pump to keep the water circulating fast enough to keep it from freezing?
and hook the battery to a solar panel. Would probably work.
It will not work. Haven't you ever seen a frozen fountain in the park? Like the other poster said, if you can pump a source of warmer water up and keep mixing it with the water that is near freezing, then yes, it can work, but he doesn't have a source of warmer water to pump and mix. The water will freeze, and likely destroy the pump too. What is going to work is a stock tank heater, and as cold as it has been getting lately in some locations, he will need a darn good one too!.