I don't know her full situation, of course, but I suspect her problem can be summed up in one word: pride. She hates to admit she was wrong and has made a mistake, and she also hates the embarrassment of facing her family and friends. The Bible says, "When pride comes, then comes disgrace, but with humility comes wisdom" (Proverbs 11:2).
I mention this because pride is a very dangerous motive for any person. Pride blinds us to our own faults; it also makes us do things that are foolish or hurtful. In addition, it makes us put ourselves first, and love ourselves more than others. Most of all, it cuts us off from God. When the devil tempted Adam and Eve, he appealed to their pride: "Your eyes will be opened, and you will be like God" (Genesis 3:5). What a temptation!
Pray for your friend, asking God to convict her of her sin and to burden her with a desire to do what is right. Most of all, pray that she will give her life to Christ, for she needs His forgiveness and strength. And if you have never done so, be an example to her by giving your own life to Christ.
Then ask God to give you opportunities to talk with her. Don't argue with her, but let her know you care, and then encourage her to do what's right, regardless of the cost.