Sometimes a person who has no faith can be very stubborn and resistant toward God, even if they claim to be open-minded. The Bible reminds us that "The heart is deceitful above all things and beyond cure. Who can understand it?" (Jeremiah 17:9).
But sometimes people who have no faith aren't as resistant to God as they seem. Your friend, for example, may be realizing just how hopeless his life is without God, and may even be wondering if he's headed down the wrong road (which he is). Either way, ask God to work in his heart and mind, and to make you a faithful witness for Christ. The Bible says, "Always be prepared to give an answer to everyone who asks you to give the reason for the hope that you have" (1 Peter 3:15).
What sets us apart from every other living creature? It's this: We don't only have a body and a mind, but we also have a soul or spirit. In other words, God has implanted something of Himself within us -- and because of that, we can know God and have a personal relationship with Him. And God did this because He loves us and wants us to know Him.
Don't get sidetracked, however, into issues that may be important but aren't at the center of our faith. Instead, urge your friend to look at Jesus Christ with an open heart and mind, because through Him we come to know God.