Resistance, I think I would take it a small piece at a time--at least, in the beginning. I don't know how dedicated she is to the WTS, but if she's "in" all the way, a long conversation will only scare her off. Or rather, scare her away from you and back into the Society's embrace. Remember that her motives are probably complex: she wants to be with her family, not just with the WTS. Don't trick yourself into thinking this will be simple. It might be, but don't count on it.
You could show agreement or understanding in certain areas, then raise concerns in others to her. I would raise these same topics to anyone I was studing with as well as there is likely to be some crosschecking between these folks behind your back at some point. (I've been there.) If you never mention the areas of concern to your study conductor, they'll know you're just a plant to get her out.
Gradually, after 6 months or so, you can begin to pull out after having second thoughts that you've been expressing to her and your conductors for a while. I would focus on the more cult-like qualities of the Society, and point out the differences between the Society and other religions that also see themselves as being Bible based. Hopefully, your bond will help her to take your doubts seriously. I would probably mention the fact that religions who see doubters as evil sound pretty cult-like, too.
Eventually, it will be time to have a real conversation, but only once you've been in it long enough to have an honest, informed opinion. And only after you've planted seeds by pointing out the problems along the way, problems she will also have to think about. Remember, you can't just point out a flaw and expect it to stick. You have to have a conversation with her about the flaws along the way.
Don't nag. Don't push too much or too often. Just give the Society's positives and negatives a fair amount of time and plant seeds. Hopefully, you'll both discover the flaws together and leave together. That's probably the best you can do. Don't lose patience or you could become a bad associate in her mind.
We can, of course, help you with this along the way. :-)