what damage could have been done with overheating the engine

My son's car. 2.5liter 4cyl. engine. the was no heat from the heater. Heat came back after adding about a gallon of coolant. The old coolant was very rust colored & I saw soome discoloration at the bottom radiator hose, but no moisture. Everything was fine for 2 days when my son reported no heat again. Coolant was very low & he said he saw a small puddle under the car. I suspect the water pump might be leaking. I'm afraid he might have overheated the engine. When I went to add more coolant, with the engine running the engine started running very rough and the exhaust was smoking (white) very heavy. It stopped running rough & smoking after I let it cool down a little. I'm wondering if water is getting into the cylinder? There's no evidence of water in the oil. If it overheated, could it blow a head gasket, or could it be a warped cylinder head? Can this be determined without taking it apart?
What should I do?
Thanks