It was, I think, because she knew that Harry and Draco loathed each other right from the beginning, and Harry still told her the truth that her son was alive. She may have done that as a show of gratitude because she was astounded that Harry saved a Death Eater, as Draco was. Harry could have simply lied that Draco was dead, and V would kill Harry, as Harry had intended. And the story-lines make it somewhat clear that what Dumbledore said ( Chapter: King's Cross) was a little doubted by Harry. Harry could save lives by dying, but there was no guarantee that he could do so by living. He could at least free families from the constant terror of losing family members by once again giving up his life. So, Narcissa may have done it as a show of gratitude to Harry or to have the chance to return to Hogwarts to find out whether Draco was alive.
