The most important thing you can do is to keep on being a friend to her -- in spite of her insults and her arrogance. God loves her in spite of her attitude -- and so should you.
Why is this so important? The reason is because your friend may deny the truth of what you believe -- but she can't deny the truth of your life. No, you may not be able to answer all of her questions; even if you did answer them, she'd probably come up with a new set of questions. But the greatest evidence you can give her of God's love and transforming power is the reality of a changed life.
I'm interested, however, in the fact that your friend doesn't cut herself off from you, although she labels you "stupid" for believing in God. Why is this? The reason, I suspect, is because down inside she's not as sure about her unbelief as she claims to be. In fact, I suspect she's really searching for God -- even if she denies it.
Pray for your friend -- and pray for yourself also, that God will help you love her and point her to Christ, both by what you say and by the way you live. May you be "blameless and pure, children of God without fault in a crooked and depraved generation, in which you shine like stars in the universe" (Philippians 2:15).