That's a different "geothermal" - the correct name is "ground source heat pump" or in your neighbor's case, a pond-source heat pump. Those exploit the temperature stability that occurs some small numbers or meters subsurface for heating in the winter and cooling in the summer.

"Geothermal energy" involves drilling down to hot rock to tap intense heat to run a turbine that produces electricity.

wouldn't it be "natural geothermal" if the pond is a naturally occurring hot spring?