RAINFALL BECAME MODERATE TO HEAVY AS THE ACFT WAS BEING PUSHED BACK IN PREPARATION FOR FLT. DURING THAT TIME, LIGHTNING STRUCK THE TAIL OF THE ACFT WHILE A MECHANIC (GROUND GUIDE) WAS WEARING A HEADSET CONNECTED TO THE ACFT. THE LIGHTNING TRAVELED THRU THE ACFT & FATALLY INJURED THE MECHANIC. WITNESSES & SUPERVISORS STATED THAT THEY HAD OBSERVED LIGHTNING IN THE AREA BEFORE THE ACDNT, BUT THEY HAD NOT OBSERVED ANY FOR ABOUT 15 MIN; PUSH BACK WAS INITIATED AFTER PHONE CONVERSATION WITH TOWER PSNL. THE AIRLINE'S SAFETY MANUAL PROHIBITED GROUND GUIDE OPN DURING PERIODS OF OBSERVED LIGHTNING.