||Great question. The way our faith and our feelings interact is difficult for any of us to understand. For some people, the intense feelings of love and forgiveness are what lead them to God. For others, the feelings come later, when they've grasped more fully what it means to follow God.
You mention two reasons for your friend's change of heart. The first has to do with her sense of guilt. Sometimes, well-meaning people follow a set of beliefs that emphasize guilt, not the love and grace of God. In those cases, it's very difficult for people to feel God's presence in their lives. They are too caught up in focusing on the ways they don't measure up to accept God's forgiveness. If that's the case with your friend, it will be tough, but not impossible, for you to get her to think differently.
The second reason your friend has given up on God has to do with her belief that God wasn't answering her prayers. Many Christians feel this same frustration. We ask God for something, then wait for him to do what we want. We forget that God knows what's best for us. I think God gets blamed for a lot of unanswered prayers that he actually did answer, just not in the way we wanted him to. God's response to our sometimes selfish and immature prayers is often "let's wait" or "this isn't best for you." For some people, these "negative" answers feel like non-answers.
So how can you help her? First of all, you're doing the right thing by keeping her in your prayers. God is the only one who can change your friend's heart. Then, show her the loving God you know through your actions, lifestyle, and faithfulness to God. Whenever possible, let her see and feel the positive experiences of your faith. When the apostle Paul stood before the Roman leaders in Acts 26, he didn't argue intellectually. He simply said, "Here's my experience." Paul disturbed the Romans with his testimony. A life lived for God speaks for itself.