All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
This girl must have repented to Allaah and concealed her sin and not have informed anyone about her sin of Zina (fornication or adultery) be him a husband or anyone else. For more benefit, please refer to Fataawa 108015 and 83573.
If this girl has repented, it is permissible for her husband to keep her as his wife as we clarified in Fatwa 88481.
Finally, it should be noted that it is not an obligation on a person to confess that he/she had committed Zina so that the Hadd (i.e. the corporal punishment determined by the Islamic Law) would be implemented on him/her even if the Islamic Sharee’ah is applied. Rather, it is permissible for him/her to conceal his/her sin and repent to Allaah. Also, not everyone who has committed Zina should be stoned to death; rather, stoning to death is for someone who has committed Zina while he/she is married or was previously married.
Allaah Knows best.