All perfect praise be to Allah, the Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.
If this took place out of her ignorance of the fact that sexual intercourse is forbidden during the state if ihram, then there is no blame upon her, and she should complete her Hajj. This is the opinion of the Shaafi‘i school of jurisprudence, and it is one of the opinions of Ahmad that was given preponderance by Shaykh Al-Islam Ibn Taymiyyah . However, if she knew that sexual intercourse is forbidden for her during the state of Ihram, then her Hajj is invalid. She has to complete her invalid Hajj, make up for it in the next year, and slaughter a camel in Makkah. She should also repent of this sin.
Allah Knows best.