We don't know that he "left her behind" but we do know that he doesn't have custody. We also know that he is now trying to get custody. If is very possible that he wanted custody all along rather than "leaving her behind" but was denied by the family court system that all to often assumes the mother to be the better parent (when it certainly appears in this case that was a wrong assumption).
If they knew what it took to control the child, they should have done it themselves.
Obviously the parents didn't know what it took, but they certainly knew what it didn't take.
Could a spanking have helped the child to understand that bad behavior has consequences?
She certainly hasn't learned anything about respecting authority with the parents going after the police.
Parenting classes are in order for the parents and maybe they ought to put their own agenda aside and be unified in their love, support and discipline of the child.