Well you have the problem of her parents. They are JWs and there is possible conflict if you get in the way of her baptism. OTOH she is 27...
Stay away from discussions about changes, doctrines, or any of that stuff. It is pointless. You made a specific point that is best not forgotten:
she is quite immature in our opinion. Like talking to a young teenager
Most likely she is being encouraged by somebody else at the Kingdom Hall. Somebody that she probably respects more than you. Sometimes people get in the mood of going around and "encouraging" people to get baptized (men trying to be Elders, woman who get pretty zeaolous, ect). You tell her about why it's not true, she will run off to this person and tell them what you said, they will tell her not to talk to you, and that is that. Remember this. While you can't be completely sure that there is somebody pushing here along, if there is that person is in control of her right now. In her immature state she will turn to that person for all of her advice. That's exactly how I was before I got baptized. A young, stupid kid who was eating out of the hands of an guy who was encouraging to get me baptized (he also was a MS trying to make Elder). I wouldn't have believed anything negative about the JWs if it was staring me in the face (in fact it was, I was on Freeminds.org many times before I got baptized! it all just rolled off of me).
If you recognize your limitations then you may have more success in dealing with her. Realize that any criticms you put out there about the Society will be met with typical responses that JWs use. Why put yourself through that kind of irritation! It doesn't work.
I think the Ray Franz book is the best option. Let Ray do the talking. More than anything tell her that you simply want her to make up her own mind and that she should be well informed. Don't tell her what she should do but rather encourage her use her own mind.