My hubby and I are both born in's. It was frightening to try and think of way to break the news.
What I did that helped lessen the initial impact of my decision was to sit down together calmly hand and hand and gently explain that what I was about to say was going to be hard to hear but that I wanted to be honest. I gave him the option that if he couldnt handle what I was about to tell him and felt that he needed to leave the relationship that I would understand. I explained calmly that I no longer believed and let this sink in. It no doubt hurted him and he cried as he realized all our jw dreams together just got blown up but after the initial shock his commitment and love for me outweighed the break up of the religious aspect.
It was the mistake that I made months later in overloading him with too much anti -WT and debating with him over the religion that caused him running to family who in turn ran to the elders. So Don't do that. Just tell her how you feel then let her get over the initial shock and leave it be. Dont go into any further details and ask that she can respect your religious differences.
Hopefully her love for you will outweigh the wt indoctrination.
Best wishes
Lostwun