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 is His slave and Messenger.
It is not permissible for a woman to talk to a non-Mahram man except for a necessity and provided she abides by the Islamic manners; for more benefit in this regard, please refer to Fatwa 81356.
If what your fiancée did is really confirmed, then she has committed an evil and grave sin from which she should sincerely repent. For more benefit on the conditions of repentance, please refer to Fataawa 86527 and 87903.
Also, she must avoid any matter that could lead her to temptation, so she should delete all non-Mahram men’s phone numbers and addresses and so forth that she may have.
Indeed, it is not permissible to send such messages that contain sexual expressions, neither to Mahram men nor to non-Mahram men. Also, it is not permissible for a Muslim to read them except for a valid purpose, like objecting to the one who wrote them and the like.
Hence, if this girl repents and turns to Allaah, then it is acceptable to marry her, otherwise we advise you not to marry her because a Muslim should be keen on marrying a religious woman as per the advice of the Prophet said: “….so, marry a religious woman, you will prosper.” [Al-Bukhari and Muslim]
Allaah Knows best.