I'm not sure you guys understand how this stuff works at large companies. There are several designated channels for receiving harassment claims, and mountains of record-keeping that go on. It's official, developed, and orderly.
All ND is saying here, likely, is that those official avenues have no evidence of allegations.
If you want to assume cover-up without any evidence, have at it, but HR people that screw this up expose the company to enormous legal jeopardy, and risk their own livelihoods. The odds are far greater that he didn't properly report it, and was let go for an unrelated reason.
He could be telling his truth here, without it being THE truth.