On her current course, your girlfriend will end up being effectively shunned by her congregation whether she is disfellowshipped or not. Besides that, right now, according to her religion, she is a candidate for destruction at Armageddon. If she marries you on the sly and lies (fools the elders) about the ongoing intimacy prior to marriage, she will still be a candidate for destruction at Watchtower Armageddon.
There is no way to win in this...unless she is willing to give up her religion which at this point will not save her anyway...and disassociate herself by letter from the religion. Then she can freely marry you. If the elders want to know why she is disassociating, she can say for private reasons...and it's no longer their business anyway.
As a non-JW she will be shunned as an apostate...but then who cares.
If she is not willing to give up her religion, your best bet is to terminate your relationship for her sake. You might want to encourage her to do some independent research into what I see as a controlling and unbiblical cult.