Frogs (and other amphibians) are capable of remaining active at very low temperatures. I am not sure what type of frog you have, the size you give is a bit too large for a treefrog, and the fact that it remains at your pond also makes me think you are seeing some other type of frog of a more aquatic variety. You don’t need to help it through the winter, it can find a suitable place to survive the cold, many species will burrow down into the muck at the bottom of the pond where the decaying organic matter keeps the temperature above freezing.