Friend is custodial parent but her ex is keeping 2 of their 4 children illegally in FL. How is he doing this?

Their divorce was final in June, she was granted full custody, and he still has the 2 oldest children (ages 17, 15). She moved out of state (Judge granted that) and the day she was to move, the 2 oldest jumped out of the car to meet their dad, something the dad concocted. How is he getting away w/ this? He also doesn't pay alimony/child support, and she wants all 4 children to be together. Anyone have ideas? Been thru a similar ordeal? Know Florida divorce/child support laws?