The only exception I would make to this is if the situation requires the LW to actively lie. What is Stacy telling the girls about their births and infancies? Does she expect others to support her story, or just not contradict it? That's a moral line I draw in a number of cases (say, knowing someone is cheating on their partner), but I freely admit to having an extra horse in this race as an adoptee. I could grit my teeth and not say anything (I just did with a student, actually, who only recently told her 12-year-old that her ex-husband is not his father), although I would probably tell HER, once, that I thought it was a supremely bad way to handle it. But if she expected me to actively repeat those lies...well, honestly, I don't think we'd be friends anymore.

(To be clear: my parents handled it in the best way possible, which was just talking about it from day one, as casually as one might discuss hair color.)