2009 SXT 3.8 emptying overflow

I haven't looked at it yet, it is my father's and he lives a ways away. He said it is like it is overheating, but no gauge or light indicators are coming up. He put a different cap on the overflow bottle, new thermostat, but it is still coming out. He said he had a mechanic then look at it, who said it probably has a bad head gasket. But they didn't do any pressure test or chemical test for exhaust gas in coolant. They just wanted to go straight to pulling the heads. With all the tests available, I told him to not go back there.
My searching hasn't turned up a lot yet, but is there any common things that causes this for this vehicle? Water pump? Does it have alum heads and needs an additive in coolant to help with head gasket seal? Anything else? Thanks for reading.