Cerberus has one job, and that job is to guard the Gates of Tartarus so that no evil creatures can escape. He's had this job for a long time, and not once did he defy his duty. Sure, he left his post that one time, but he never intentionally set someone free. Not that he ever wanted to.
But what if somepony was cast into Tartarus through no fault of their own? And what if that somepony is a pony that Cerberus really cares about? Is there a protocol for this scenario? Alas, no. It's never happened before. So, the big question remains:
What now?