When your friend asks you to prove the existence of God, ask him to prove that his wife loves him.
Proving the existence of God is a strange task, along the lines of proving that my wife loves me. However, we are called upon to do it from time to time by our non-Christian friends. “How can you be so certain?” they ask. “What if you’re just imagining it all? How can you prove to me that this God of yours exists?”