My Friend's Ex Won't Let Her Leave Without Her Baby - Florida

Background info: My friend married a guy and they had a baby boy. (they're both Brazilians but he has American citizenship and she doesn't). She came to the US a few months ago to introduce the baby to her (ex) husband's parents but their relationship isn't working out so she wants to leave in January. The baby's dad says he will bring the baby to Brazil in March but her Visa expires before that so she wants to take the baby with her next month, but he said he won't let her because he's the dad, but she's also scared he won't take the baby back to her in March. Question: is there anything she can make him sign to make sure he will take the baby to Brazil back to her? In Brazil we would call it a "procuração" and I looked it up and it translates as POA (power of attorney) but when I read the definition it doesn't sound like what we're looking for.