The Power of Let

"While reading a manuscript by Mrs. John Peters, I was intrigued by one episode in particular. Losing his footing in the bathroom, her husband struck his head on the ceramic soap dish. One ear was almost severed and he was bleeding profusely when Mrs. Peters heard his cries and came to his aid.
Despite her shock at the sight of so much blood, the Spirit took over and enabled her to speak with authority. She heard herself saying, "Let the bleeding stop immediately. Let there be no infection. Let there be no pain, Let there be no scaring."
Mrs. Peters made no comment on the experience other than to report that, gloriously, the bleeding stopped. There was no infection. Almost no pain. No scars. But something about these "Lets"stuck like glue to my mind. I realized that it was the same word God had used in creating our world "Let there be light,"
        Jesus to His disciple:
                    Let you light so shine before men, that they may see your good works, and glorify our Father which is in heaven. (Matthew 5:16)
                    And if the house is worthy, let your peace come up on it; but if it is not worthy, let your peace return to you. (Matthew 10:13)
        Paul used it too:
                    Let this mind be in you, which was also in Christ Jesus. (Philippians 2:5)
What, I wondered, is the significance of this word for us? Author Harold Hill gave me the missing insight. "'Let' is a word of tremendous faith with volumes of meaning poured into it, " he told me. "It assumes the total love and good will of the Father. It assumes that heaven is crammed with good gifts that the Father desires to give Hid children. The 'let' is saying, 'Father, I give to You permission to do so-and-so for us down here on earth. I allow it.'"
It also assumes an almost preposterous humility on God's part-that He should wait for our permission to bestow wonderful gifts on us! How amazing!
Worlds of meaning behind this three-letter word...let."       (Marshall, A Closer Walk, 1986, pg 75-76)

After reading this passage, I had to buy the book for myself. The Power of 'Let.' But can the power of 'let' override God's will? Is the power of 'let' supposed to take time? When God said, 'Let there be light,' it was automatic. But I have used this word in relation to a friend's healing almost every day since I heard this a month ago, and she doesn't feel any better. Is it lack of faith? How much faith do I need to believe that God's the only one who can heal her? Is this where God says, "Thank you for believing in me, but it's not my will that she be healed." Then how do we know when the power of 'let' really has power?
It's still a profound passage when referencing the usages in the Bible though.