He and I already talked about it last night. His story checked out - he's mined over 5000 sand, which is plenty to construct that much TNT. The TNT was placed before or the day of the revision of the griefing rules. I'm not sure if the gunpowder he traded for was legit, but it's possible if someone stood in front of a harvester all day long. Besides, that wouldn't really be his problem (only would be if it was obviously impossible to acquire the resources, such as 64 diamond blocks or something).