Praying for this woman is the most important thing you can do -- because only God can break through the hardness of her heart and open her eyes to her need of Him.
Surely one of life's greatest puzzles is how we keep doing things that inevitably hurt us or even destroy us -- although we ought to know better. Even when we've done them before, we keep going down the same road, hoping somehow that this time it will be different. But it never is -- because the consequences of unwise or sinful actions never change. The Bible warns, "Even as he walks along the road, the fool lacks sense" (Ecclesiastes 10:3).
Why is this? The reason is because we each have a "disease" inside us -- a "spiritual disease" the Bible calls sin. And one of sin's greatest consequences is that it blinds us to our own sin. This is why the Bible warns not to be "hardened by sin's deceitfulness" (Hebrews 3:13).
Do all you can to be a friend to this person, asking God to show His love for her through your life. Trust Him also in time to open the door to talk with her, discovering not only why she's turned her back on God, but also telling her of His great love for her -- and His power to change her life through Christ.