Over the years, I've heard many people say the same thing your friend said about heaven. To them, heaven isn't real, and they think people who believe in heaven are just burying their heads in the sand and trying to escape from this world's problems.
But I'd contend the opposite is true, and that they are the ones who are burying their heads in the sand and escaping from reality. We weren't meant for this world alone; we were meant for eternity. Death is not the end for us; when we die our souls will live on -- and refusing to face this is escaping from life's ultimate reality.
How do I know heaven is real? I know it first of all because in our hearts we all sense there must be something more than what we see around us. As the Bible says, God "has also set eternity in the human heart" (Ecclesiastes 3:11). But I know it most of all because of Jesus Christ. He came from heaven for one reason: To break down the barrier between us and God, and open heaven's door for us. The Bible says, "For the wages of sin is death, but the gift of God is eternal life in Christ Jesus our Lord" (Romans 6:23).
Pray for your friend, that God will open her eyes to His truth. And pray for yourself, that God will use you to show her the reality of heaven -- both by what you say and by the way you live.