Did you know that if your radiator and overflow are not full or have air bubbles your ac will not blow cold? If a mechanic charged the ac and the compressor is working I don't think it is fair to blame the mechanic he should have a pressure gauge and should have used the right pressure fittings. Make sure the radiator and overflow bottle are filled up to the top line and up to the top of the radiator and make sure the fluid is fresh. If it is old flush and refill with fresh antifreeze. Shut it down wait for engine to cool and air pockets are gone and refill any missing fluid this may solve your problem. Also the mechanic should know if your compressor was shot.