Speaking as someone who once rejected the notion of there being a God, I might be able to offer some insight. My biggest hurdle in coming to understand there was a God was the notion of such evil in this world. Most people that reject there being a God have the same problem, I believe. We see (or in my case saw) it as impossible to believe in a loving God that allowed evil to happen in this world.
The way I would approach that particular subject is this way. We need to get those people to first understand that we all have free will. The most loving thing God has given us is the ability to accept or reject all that we are given as gifts from Him. I don’t think I fully understood that until my kids got to an age where they had to start making some really tough choices on their own. I knew what I wanted for them, but I had to sit back and watch them make their own choices, whether I agreed with them or not. Whatever choice they made, however, didn’t affect my love for them one bit.
Secondly, we must get those people to consider the possibility of there being a Heaven. They don’t have to fully believe at first-they need only to consider it. Once we allow ourselves to consider an eternity with our Lord, it makes the concept of evil in this world make more sense. Atheists are still longing for Heaven-they just don’t see it. They can’t get past the thought that it isn’t in this world. But when those that don’t believe express their desire to not have evil in this world, what are they really expressing other than the notion that they too want a Heavenly reward? They want what we want, only they want it in this life. Getting them to see it is possible, but we must work and prepare our souls for it to come later is the challenge we face.
Lastly, on the issue, we need to get those that do not believe to see the role they can play in not allowing evil to take over. Jesus Christ came and taught us how to love one another, just as He loved us. We need only follow His lead, and love each other just the same. We are sinners, and fall far short too often. But we must keep striving for what Jesus taught. Atheists, on that point, need to see that we are all on the same team. We want the same things. Non-believers see Christians as standing in the way of the utopia they envision. Really we all want a better world then we currently find ourselves in. Jesus is exactly what He says He is: the way, the truth, and the life. Have your friend read His sermon on the mount and see if there is a moral argument against it. Some of us already know there isn’t one. No one has a “Jesus problem”-Jesus hasn’t changed in 2000 years.