If you marry before anyone knows of it then there cannot be any suggestion made that she should end the relationship. Marriage (even to an unbeliever) is "sacred". However, she will be "investigated" by two elders to determine if there is cause for a Judicial Meeting over any "sin". Obviously, your fornicating (your word, I believe) is a "sin" in the JW rules (and elsewhere to many). Fornicating "often" without her ever confessing and ceasing her 'wicked way" will most definitely result in her being disfellowshipped. From recent Elders School, a person cannot be deemed repentant "immediately" once they are caught if they carried on the offense for an extended period of time. (Of course King David WAS repentant immediately when Samuel confronted him about Bathsheba.)
The only way she can avoid being disfellowshipped is by avoiding meeting with the Elders (which would surely result in her losing her pioneer position which she will lose anyway just by marrying an unbeliever), or by meeting with them and DENY DENY DENY any fornicating or unclean conduct and even any overninght sleepover even if there was NO fornicating. (Staying over night alone with a person of the opposite sex is circumstantial evidence of fornication). If she denies anything immoral (or in conflict with JW rules) ever occurred then the Elders cannot take any Judicial action as there are NOT "two witnesses" (or a confession) to wrong doing. You two will have to determine if she can lie her way out of it.
Note there is a scripture in Psalms which says to confess your sins to Jehovah alone. You might use that to convince her that she does not owe the Elders any explanation about your "sinful" relationship.
Good luck. You're involved with a person who won't leave their religion but sneaks around and breaks the most ardent rule that the religion has.
