Can a Non-Custodial parent refuse to give up a child in a different state?

My sister is serving her ex and her child's father with custody papers from the state of california. Currently, the child is with her dad in oklahoma. Our Paperwork states that He is to return the child IMMEDIATELY to my sister. Can he refuse and get away doing it because our paperwork isnt from the state of oklahoma? he is driving out next month to pick her up, and serve him. The child is also under jurisdiction of california (uccjea). Any advice, or propable outcomes she should expect?