Most common heating issues are the rusted water body as mehran parts tend to rust more often.
if the radiator is already rusted, suggestion is to get it serviced and if the condition is bad replace it. The guy doesn't have much info about cars, so he cant perform the TATRI method easily.
Secondly, inspection of the water body should be done.
thirdly, i am little confused about the last line he has written
"My mehran 2012 fan is direct it gets quick heat and quick cool the mechanic is saying it’s because of the radiator as i am new to cars i dont know about the radiators and if I should replace what should I replace it with copper or other and remember that without ac it remains in between C H.
it needs to be clarified.
And last but not least, maybe the compressor is putting load on the engine.