My cousins car keeps overheating when it is.

It sure would be nice to know the engine size. First, is the radiator fan turning on when the engine warms up? Second, feel across the entire radiator to look for hot or cold spots. If one part is cold, the cooling tubes may be blocked. If part of it is really hot, scratch the cooling fins. If they crumble away easily, they are rotten and will not disperse the heat to the air. Clues to these are the overheating will be less severe if you run the heater on one of the higher speeds.

If there has been any loss of coolant, you might suspect a leaking head gasket. Combustion gases can leak into the cooling system and pool under the thermostat causing it to remain closed. That's more common on the 2.5L engine. If that's the engine you have, did you burp the air out after replacing the thermostat? If you did not, and the overheating causes the cylinder head to warp, doing the work yourself will be the most expensive way to fix the car, not the cheapest.