As a former atheist,the one thing I could not find an answer for was the Beginning of life. I could explain evolution and the Big Bang but could not tell you where the gases and space particles came from nor where amino acids came from.
Depending on how strong an atheist, and yep, there are some who sorta sit on the fence that are not “rabid” in their stance. You might want to get this person to focus on these points that I had trouble with. Do Not let this person change the subject on you though, keep him focused on just these points.
Then though, as any atheist worth his salt will say,“Then where did GOD come from?” be prepared to give an answer for that and also be aware that it will do a “low” fly over on him.