What is the ruling of Islamic law regarding a husband saying to his wife via WhatsApp: "You are divorced, O so-and-so," knowing that the wife is a mother of a 70-day-old child and has experienced one menstrual cycle after her postpartum bleeding?

Writing a divorce (talaq (definition)) through the phone or other means is a kinaya (an indirect expression) that depends on the husband's intention. If he intended to effect it, then it occurs; otherwise, it does not. As for the divorce of a menstruating woman, the majority of scholars hold that it occurs. The spouses are advised to avoid problems and disputes to ensure family stability, especially when children are present.