A Line Mechanic reports deferring an APU that had made a loud bang and then shut down; sending debris out of the APU exhaust pipe onto the ramp and right engine pylon.

Narrative

My Lead Mechanic told me to meet the flight crew and ramp personnel at Aircraft X; a B717. When I got there; debris was on the ramp at the rear of the aircraft. The Captain said the APU made a loud bang and then shut down. I informed my Lead Mechanic over the radio that I was opening the APU access doors. About a minute later; the Lead showed up. The left and right access doors were open and he helped me open the lower center APU panel and the access panel for inspecting the APU compressor. He then asked if I would start MEL procedures to [make inoperative] the APU and contact Maintenance Control. As I was doing that (in the jetway); I heard someone (the Shift Manager I think; but I am not sure) advise over the radio to inspect the Aft Accessory Compartment and APU enclosure for damage. The Lead Mechanic then came on the aircraft into the cabin and headed aft. When he returned; he said there was no damage in the Aft Accessory. He then left the jetway. I then asked him to verify the APU Air Inlet Door was closed. He said it was closed and he had removed debris that had come out the APU exhaust pipe onto the right engine pylon and that he was done outside. I completed MEL procedures and gave the Captain the Logbook.I was notified by phone by ZZZ Station Manager. I assumed the Lead Mechanic had found no damage in the APU enclosure. NEVER ASSUME! The person that signs the Logbook is the one responsible. This incident is my fault.