I think a gentle reminder to always respect others' beliefs (even if they seem ridiculous to her or are different than what she has been taught at home) is in order. Be it religious beliefs, Santa Claus, or whatever. If another kid asks her if she believes in fairies, or Santa Claus, she can say no. But there is really no reason for her to bring the topic up, or to elaborate on her own beliefs or get into an argument about it. The same social norms apply to adults. Unless I know someone well and they are a good friend, I'm not even going to bring up the topic of religion, for instance.