What does Rose say?

Just before the scene where the other mothers won't let their kids come to Tommy2's birthday party, we see Rose talking to them. The implication is that Rose has told them that Tommy2 is a clone. My question is, how would she know that? Rebecca and Ralph were the only two who knew about it and there's no reason either of them would tell Tommy1's one-night stand about it. The only thing I can think of is that if he did lie about having met her only the day before and they knew each other when they were kids, she might think that Tommy2 looks exactly like Tommy1, but there's no reason why Rebecca couldn't have become pregnant before he died and their son could just bear a very strong resemblance to him. Anyone have any idea? It seems weird that a former lover of the deceased seems to have such insight into the reproductive goings-on that occurred after his death.