OK.. you may just have air pockets in the system as a result of replacing the thermostat. Have him make sure he followed the procedure HERE.
If the system has been properly bled and it continues to run hot, then you need to make sure the radiator
fan is running. If the gauge is near the red and the fan is not on, see if the fan comes on when the A/C is turned on. If it still doesn't come on then it's possible the fan itself is bad. If it does, then it could be a computer or cooling temperature sensor problem.
When the car is hot (close to overheating) have him CAREFULLY feel both radiator hoses, and both heater hoses. They should be all close to the same temperature. If not, this indicates a restriction somewhere.
Also if the fan is on, the air it is pulling through the radiator should be HOT... not cool and not luke warm. If the radiator hoses are both hot and about the same temperature, but the air from the fan is cool or luke warm, this indicates that the radiator is likely bad (corrosion inside coats everything and prevents the heat from transferring from the liquid to the radiator properly). Also if the air starts off hot and then cools off very quickly (and the engine is still overheating or close to it) this also indicates a radiator problem.
Check the engine oil.. make sure it is brown or black, and has no signs of a white or cream color, as this indicates water (or coolant) in the oil.. which is usually the result of a blown head gasket
. This can cause an overheat, and this can become the product of an overheat as well.
It is also possible for a head to leak in a way that exhaust
gases enter the cooling system, but coolant doesn't enter the oil. When the engine is cold, make sure the radiator is completely full and that the radiator cap is installed. Start it up. It should take a minute or two for the upper radiator hose to become pressurized (stiff). If this happens immediately, this is usually a case of a head gasket problem. It is possible for this to happen, but not so rapidly -- but testing for it might require emissions analyzing equipment. Just because it passes the 'driveway' tests I just gave you, doesn't necessarily mean the head and head gaskets are good.
Also too little or too high a concentration of antifreeze can also cause overheating, especially on a very hot day. If using full strength antifreeze, you're supposed to mix it with water (50%/50%). If using the premixed stuff, then don't add any water.
He needs to start with bleeding the cooling system though. some of these motors have two bleeders, and some only have one.. Some use a standard head screwdriver, and some use a 7mm socket. When the engine is cold, make sure the cooling system is full and open these bleeders until only coolant comes out of them. Carefully close them (very easy to break so barely crank them tight) and follow the procedures listed in the link above.