He needs a liscened contractor. Is he having any other problems electrically?
It very well coud be how the service is set up for the condos, He could either have a voltage problem caused by the loud bang at another unit, which could have caused bad connections to his unit.
He actually coul have a lot of other things wrong not getting proper valtage phase to phase or phase to neutral.
A loud electrical bang as you called it means something shorted somewhere. And the inrush current could trip the main.