Is it permissible for a woman to commend a person she was engaged to, then separated from, to God, hoping that God would return him to her?

There is no harm in supplicating for someone to be preserved and to return; supplication is legislated as long as it does not involve sin or severance of kinship ties. The Prophet, peace and blessings be upon him, entrusted individuals to Allah and supplicated for their preservation, saying: "I entrust you to Allah, with whom deposits are not lost," and he said: "Indeed, when Allah عز وجل is entrusted with something, He preserves it."