do you guys go to church? have the elders pray over her? and have YOU told the voice to leave her alone in the name of Jesus. If you have the Holy Spirit inside of you and she is a believer you can pray together that the voices leave her alone.
i've seen them whispering to others. the best defense is reading the Bible. here are scriptures about donning the armor of God for we do not battle enemies of the FLESH but forces of darkness in the spirit realm. Make sure she has confessed Jesus as her Lord and savior, attends church, reads her Bible and is baptized.
In addition asked if she has been abused by someone in her life, most girls have been, at least 80% and most don't ever tell anyone. Abuse makes holes in our shield of Faith that allows the whispering to be heard.
Dear Lord we pray for our brothers and sisters who are suffering from spiritual attack. please strengthen their shields of faith, open their hearts, minds, eyes and ears so that they may see the world as YOU see it and so that we as ONE BODY move in a manner that shows the whole world that GOD LIVES!
Ephesians 6
The Whole Armor of God
10 Finally, my brethren, be strong in the Lord and in the power of His might. 11 Put on the whole armor of God, that you may be able to stand against the wiles of the devil. 12 For we do not wrestle against flesh and blood, but against principalities, against powers, against the rulers of the darkness of this age,[c] against spiritual hosts of wickedness in the heavenly places. 13 Therefore take up the whole armor of God, that you may be able to withstand in the evil day, and having done all, to stand.
14 Stand therefore, having girded your waist with truth, having put on the breastplate of righteousness, 15 and having shod your feet with the preparation of the gospel of peace; 16 above all, taking the shield of faith with which you will be able to quench all the fiery darts of the wicked one. 17 And take the helmet of salvation, and the sword of the Spirit, which is the word of God; 18 praying always with all prayer and supplication in the Spirit, being watchful to this end with all perseverance and supplication for all the saints