96:5 Work may not be done before havdalah (even work prohibited only rabbinically – Mishnah Brurah 299:32; work may be performed after the version of havdalah recited in Shemoneh Esrei – OC 299:10). Therefore, if women (or men – apparent from ibid.) need to turn on lights or perform other acts of labor before havdalah, they must first say, “Blessed is the One Who separates between the holy and the secular, between light and darkness, between Israel and the other nations, between the seventh day and the six days of labor. Blessed is the One Who separates between the holy and the secular.” If yom tov starts after Shabbos, we conclude, “the One Who separates between the holy and the holy.” (One must still hear havdalah recited over a cup to be able to eat – MB 299:35.)
96:6 One who is late to daven Maariv after Shabbos, or who extended his third Shabbos meal into the night, is allowed to direct another person to perform labor for him – even another Jew who already davened and said the form of havdalah that appears in the Shemoneh Esrei. The one who asks is allowed to benefit and eat from the other’s work even though when he bentches he will still recite the paragraph for Shabbos.