One time my then-teenage daughter asked me if she could attend some local church group with some new friends. I told her 'fine with me, as long as we sit and I tell you what to expect, and we review what happened.' We had several good talks. As her association with them progressed she eventually decided to have nothing to do with them.
Of course the effectiveness of any approach will depend on how indoctrinated she already is. But I sum it up this way: Education, education, education.
Education: Study a set of 'We are the Only True Religion' sects/cults. They love bashing other groups. She may notice, perhaps later, that the JWs exhibit the same traits, claim the same things.
Education: Read up on Logical Falacies. Witnesses use a variety of flawed logic to convince those that want to believe. Study things like circular logic, strawman, ad hominem, etc. Learning to spot those will help her see - if she wants to see.
Education: Take her straight to the beliefs that their recruiting methods hold back. Show her what the JWs think she is not ready to see. Explain that all babies, with the exception of those belonging to JW mothers will be destroyed at Armageddon (that one seems to work pretty well with women). Take her to Quotes' web site and show her, in JW publications, the loony things she will be expected to believe - without reservation.
Tough spot - good luck.