A sadly departed, ex colleague and very good friend of mine told me a very similar story to the OP's. He was flying an RAF Nimrod over the Atlantic one stormy night when the aircraft was struck by lightning. A football sized lightning ball appeared between the two pilots, drifted rear wards through the cabin then departed up inside the fin with a loud bang. He didn't personally see the ball because he was temporarily blinded. The aircraft was diverted to Newfoundland. The following morning, during an aircraft inspection, it was discovered the the front of the MAD boom had been blown off.