You can't just become a Jew without having some level of belief, ideas or customs, otherwise I could call myself a Jew. All I was saying is that she may find she does not want to believe in some aspect and choose not to be Jewish.
For instance, you say that she stopped being a JW because she accepted the Jewish calculations. She may later decide that she no longer accepts this and believes the JWs have it right and return to being a JW. Or it may be too hard for her to deal with the shunning from her own family and return to the JWs. It doesn't matter if you tell me that she doesn't have to accept the Jewish calculation or not in order to be a Jew, in her own mind, it may be an important issue to her.
You later explain why she wants to become a Jew. The point is, she may decide these are not valid reasons. Her husband might decide to become a Mormon and she's happy to go along with that to keep family peace.
So tread lightly before you start speaking about what this woman is doing. She is joining is new culture because she is marrying a Jew and they plan to start a family. She wants to be a part of a culture to raise her children in it. She may end up being an atheist, but she wants to raise her children in such a culture that will allow her children to be free and safe to do the same. The Jehovah's Witness culture won't allow for that, but ours, the Jewish culture will. She understands that. She is studying who we are, but not like Listener has mistakenly come to the conclusion of adopting some central set of beliefs--of which there are none.
Again, it doesn't matter if she has to accept a central set of beliefs or not, whatever it is that you are feeding her, she simply may choose not to accept it.
You may think that being a Jew is simply the best and you appear to be tickled pink that this young lady is no longer a JW. You said initially that it was because she realised the date for the memorial was wrong. There are ex JWs that do return because the religion has a strong hold on individuals. It may happen at any time, just as easily as she stopped being a JW, she may return. It can result in a great deal of unhappiness within a marriage.