My friend recognizes she is angry with God because she prayed God would keep her husband from beating her and God did not stop him. What do I say?

First, respond with empathy. Anyone who struggles with abuse needs to know she is cared for.

Secondly, don't feel you have to defend God. Generally a theological explanation on humans' free will doesn't answer the question. The question comes from the heart that hurts, not a mind that doesn't understand.

Thirdly, your greatest explanation of Christ's concern is your personal caring. People will come to believe God cares when they see us care. You might look for the opportunity to ask the question, "Who loves you the most? More than anyone else, who most cares for you?" When the abused person responds you can comment along the following lines:
a. "God's love is often expressed through people who have the capacity to feel your hurts and the energy to lend a helping hand."
b. "I have always believed God's love is far greater than any human's love even though I have not always felt that way."
c. "I too have experienced the silence of God in response to my prayers. I found myself confronted with the decision to mistrust God or to keep hoping and praying."
d. "Jesus Himself did not have all of His prayers answered."

An evangelistic appeal will be appropriate when your friend shows a continuing interest in spiritual things. At some time you will want to ask, "Are you able and willing to trust God with your past hurt?" When she says "Yes", you will be able to pray with her and invite her to take another step closer to God's forgiveness. Your caring and comments are stirring her faith in God.

(For a greater explanation go to http://www.missionevangelism.org/unanswered.html and read a section on unanswered prayer from the unpublished manuscript entitled, Yes and…A Thinking Person's Encounter With God by Lyle Pointer and Brint Montgomery).