If a man says to his wife: "If you do this, you are divorced," and the action occurs, does the divorce take effect? And what is the waiting period (iddah) if he divorces her while she is menstruating?

What the man said to his wife is considered a conditional divorce (talaq mu'allaq) that takes effect if the condition is met. Divorce during menstruation is forbidden by Islamic law, but it still takes effect. As for the waiting period ('iddah) for a divorced woman who menstruates, it is three menstrual cycles. The menstrual cycle in which the divorce occurred is not counted among these three, because Allah Almighty commanded three complete [menstrual] cycles.