depends on what the mech told your friend. Did the car overheated while idling? did it overheated during driving over 40 mph? if the first, I'd suggest it was the fan switch, and if mech was not to replace it, then it's not his fault. if it's the second, then it probably is his fault. Did you make sure the car had coolant in the radiator? How fast were you going? when was the filter/oil changed? haw fast did it overheated and what was the temp and for how long before you pulled over?