What is the ruling on sending a nursing infant to his divorced mother in another country, given concerns about her raising him, or settling for limited visits with her, or not allowing her to see him at all?

Scholars have stipulated that custody belongs to the mother if she is religious and of high moral character, and capable of fulfilling this responsibility. Since the mother in question is not fit for custody, it is not permissible to send the child to her for upbringing. There is no harm in her visiting her children, provided she does not travel without a mahram, and the father can accompany the children to visit her. It is advised to focus on inviting her to Allah, and to return custody to her if she repents.