And my response was this: Of course God exists, that's not the question that is bothering you. What you are needing isn't apologetics, an argument for the existence of God. What you are looking for is a compelling vision of Christianity.
To be clear, I'm not saying this response is wholly satisfactory. Nor is it an answer that fits everyone. But it fits some people, like my friend. And for these sorts of people my answer is gesturing toward something I think is important but often missed in discussions about "doubts."
Specifically, a lot of people have a deep, often unarticulated, feeling that the world is mysterious and that "something" is going on with the universe. There is something transcendent tied up with and mixed up with truth, beauty and goodness. There is something sacred and hallowed. We suspect there is a God.
What is lacking, generally, isn't this experience of sacredness, transcendence or awe but a compelling vision of who or what God is and how we might find our place in the unfolding drama and adventure. The issue isn't really about if God exists but how our feelings of being haunted by transcendence can be translated into a compelling, vital and energized mode of living, working, creating and loving.