Paja, I feel for you and your girl! I don't envy you at all. This could be the fight of your life.
I think practically speaking she needs to examine her religion and see if it stands up under scrutiny,
I definitely agree with El blanko.
I'd just say that using the bible could be great because I think she can only be hoisted by her own petard. She probably simply won't listen to arguments of apostates. But using the bible could probably help to open the doors because it could make her think. Using the bible won't frighten her and in the end it could lead to a newfound freedom of hers as she might realise that making decisions for herself isn't wrong at all. (At the moment it sounds like she's not giving herself much credit in this.) This discovery then might help her to stand by her man.
I for example would suggest to give 1. Cor. 13:5 a try: "So faith, hope, love remain, these three; but the greatest of these is love."
As Jesus points it out here it's good to believe in something and it's good to hope for something but Jesus says it's way better and more important to love. Having faith (and acting according to it) obviously is not as important to Jesus as to live love i.e. to make decisions with a loving heart. That of course makes acting loveless but faithful an absolute "No no" in Jesus' eyes. Jesus is considered a great teacher. She will admit that. That could be a starter.
Using this method (discussing with her on her grounds) of course for you means to read as much as possible i.e. the bible, here and all the great books about the subject, simply anything that could get you "armed".
Paja, good luck and please keep us informed