The Bible tells us to avoid being judgmental or harsh toward others, and not to criticize someone unless we have a sound reason to do so. The Bible says, "A man's wisdom gives him patience; it is to his glory to overlook an offense" (Proverbs 19:11).
However, the Bible also tells us that we have a responsibility to help others in their spiritual walk with God. One way we may do this is by encouraging them to face any sins that still have a grip on their lives. God is not only displeased when we sin–He is also dishonored. When we sin, we give people who don't believe in God an excuse to keep on in their unbelief. The Bible says, "let us consider how we may spur one another on toward love and good deeds" (Hebrews 10:24).
Pray for this person, and ask God to help you confront her with this problem. Do so privately, however, and with gentleness, love and humility. The Bible says, "if someone is caught in a sin, you who are spiritual should restore him gently. But watch yourself, or you also may be tempted" (Galatians 6:1).
If she is a believer, she needs to realize that what she's doing is wrong. And if she is not, she needs to face her spiritual condition and yield her life to Christ.