Same question could be asked of Paterno
Nevertheless, Paterno timely passes along these 2nd party allegations to the AD who has the duty to investigate and report. We don't know how often Joe followed up with the AD nor do we know what the AD told Joe about the progress of the investigation. But everyone is certain that Paterno should have called the cops with unsubstantiated hearsay.
Should Paterno have done more? probably. But the truth is that right now we and most of the commentators commenting don't know what Joe knew when this was playing out in real time. That doesn't seem to stop anyone though.
Let's back up to when the Dad told Joe. Joe's advice to the kid should have been "you need to make a report about what you saw."