I just felt finished reading Combating Cult Mind Control by Steve Hassan. If you haven't read it yet I would encourage you to do so. At one time he was in a cult, so he really understand what is going on with the thinking. He has helped hundreds of people to break free of cults. In the first edition of the book he didn't mention JWs, he didn't know they were a cult, but the latest edition specifically mentions them.
Basically, a direct approach where you try to prove the religion is wrong just never works. Never, ever. Even if they have doubts, a direct confrontation just causes the defenses to kick in and they will shut down. This is why they talk so much about us "mentally diseased apostates" it's a phobia instilled into every JW, saying anything negative about the organization causes the fear to come up and you will lose them every time.
It's better to work on building the relationship, gaining trust. Every person that is in a high control religion has two personalities, the person they are because of the religion and the natural personality, try to connect with the natural personality. Talk about things outside the the religion, what she wants to do in her life, her dreams for the future, that kind of thing. Bite your tongue if you have to, but don't talk about the JWs, especially at first. When you do get to that point, ask questions to get her to think, and so you know what her doubts are, work on that.
Good luck
Lisa🌹