With the engine cold. Remove the radiator cap. Start the engine. Put your palm over the radiator fill hole. See if it feels like a pulse with pressure behind it. Have someone else rev the engine and see if the pressure increases. Take note of your coolant level. When you start the engine. It should drop down if it needs it. Leave the cap off, watch to see when the thermostat opens. Note the coolant level. Once the thermostat opens. Put your palm back over the hole to see if you feel any difference. Once you do this add coolant as needed. Put cap back on. Watch the temp gauge to see if if still pegs out. I'm thinking you have air in the system. The idiot mechanic who removed the cap and the coolant hit the ceiling did so with a hot motor.
I've seen that happen so many times. Lucky he didn't get burnt.