You've got to realize that an atheist's perspective is by definition irrational and not therefore subject to "rational argument".
What I mean is this: Atheism is a universal negative which is a logical contradiction...and to affirm a logical contradiction is irrational...therefore there isn't a thing in the world that you can say from a "human" perspective; because any argument you present will be filtered from a view that has irrationality as its very foundation.
What you can do however is to speak the word of God...let that word do God's work. It has been my experience that people forget what I said...but once they hear the scripture it gets into their head and heart and can really "nag" at them.
Your friend probably gets quite agitated when you start quoting scripture (I've had people come completely un-glued on me)...but that's a good thing...because it tells me that the Holy Spirit is doing His job of bringing conviction to a hard heart.
If that wasn't the case, they wouldn't be so bothered by it, would they?