She may not accept your views but instead go with her own views.
Never the less, if she claims to be a Christian she ought to listen to God's views.
I think you ought to show her some scriptures, if she is not convicted by them then I'm afraid you may have a bigger problem on your hands - she may not be saved.
1 John 1:6
If we claim to have fellowship with him yet walk in the darkness, we lie and do not live by the truth.
Here are some scriptures which you should use to highlight to her that what she is doing is very wrong in God's eyes - and he sees all.
A person can go to Chruch everyday of their lives because that is how they were brought up, it doesn't mean to say they are saved.
1 Corinthians 6:18
Flee from sexual immorality. All other sins a man commits are outside his body, but he who sins sexually sins against his own body.
1 Corinthians 6:9
Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals,
Ephesians 5:3
But immorality or any impurity or greed must not even be named among you
Hebrews 13:4
Marriage is to be held in honor among all, and the marriage bed is to be undefiled; for fornicators and adulterers God will judge.
Galations 5:19
The acts of the sinful nature are obvious: sexual immorality, impurity and debauchery;
1 Thessalonians 4:3
It is God's will that you should be sanctified: that you should avoid sexual immorality;
Colossians 3:5
Put to death, therefore, whatever belongs to your earthly nature: sexual immorality, impurity, lust, evil desires and greed, which is idolatry.
1 Corinthians 6:9
Or do you not know that the unrighteous will not inherit the kingdom of God? Do not be deceived; neither fornicators, nor idolaters, nor adulterers, nor effeminate, nor homosexuals,
Ephesians 5:5
For this you know with certainty, that no immoral or impure person or covetous man, who is an idolater, has an inheritance in the kingdom of Christ and God.
1 Corinthians 6:13
"Food for the stomach and the stomach for food"--but God will destroy them both. The body is not meant for sexual immorality, but for the Lord, and the Lord for the body.
Hebrews 12:16
See that no one is sexually immoral, or is godless like Esau, who for a single meal sold his inheritance rights as the oldest son.
I totally agree with Forealzchola, if she insists on continuing in this sin, kick her out. By allowing her all her to stay, you are in fact condoning what she is doing!
Luke 17:3
"Be on your guard! If your brother sins, rebuke him; and if he repents, forgive him.
Galations 6:1
Brethren, even if anyone is caught in any trespass, you who are spiritual, restore such a one in a spirit of gentleness; each one looking to yourself, so that you too will not be tempted.
Matthew 18:17
If he refuses to listen to them, tell it to the church; and if he refuses to listen even to the church, treat him as you would a pagan or a tax collector.
Matthew 18:17
Now we command you, brethren, in the name of our Lord Jesus Christ, that you keep away from every brother who leads an unruly life and not according to the tradition which you received from us.
1 Corinthians 5:11
But now I am writing you that you must not associate with anyone who calls himself a brother but is sexually immoral or greedy, an idolater or a slanderer, a drunkard or a swindler. With such a man do not even eat.