You should not tell her lies; you should tell her the truth, because telling her lies does not serve any purpose or benefit her in any way, or make her happy.
With regard to prayer and fasting, there is no kafarah to be paid, and there is no sin on her, because she has become like a child. So long as she cannot tell what is what, and her feelings and mind have changed, she is not obliged to pray or fast, because she is not held accountable for that. She is now like a little child who is free of responsibility and there is no sin involved, in sha Allah.
And Allah knows best.