Is a person considered an infidel (kafir) if they say to someone who has been wronged by another, "Cast a spell on him," even if the wronged person does not act upon that suggestion?

What you said is a terrible abomination and an أمر (command/matter) involving something forbidden, and you must repent to Allah Almighty for it. It is not disbelief (kufr), because not all magic is disbelief; rather, some of it is disbelief, and some is not.