See if you can get her on why the GB have authority - see if you can get her onto the subject of 1914. Then ask her what date they start counting from to get to 1914.
The date is 607 BCE - just see if she knows what year of Nebuchadnezzar that was (she should say 19th). FYI Watchtower changes it to the 1st year of Nebuchadnezzar. They say his first year was 624 BCE and his 19th year/aka his 1st year was 607 BCE.
The reason I suggest this is because if you look on the official website and go to the 2 videos which purport to be an explanation of their twaddle, what you notice is they do not mention how they arrive at the start date. They waffle on about 1914 being the end of the 2520 years. But they don't explain what the starting point is. Maybe it'll start her questioning if the subject interests her.