When is the women entitled to half of her dower?

When she is divorced before the consummation of the marriage. When the woman is divorced prior to consummation of the marriage, she is entitled to half of her dower.

And if ye divorce them before consummation, but after the fixation of a dower for them, then the half of the dower (Is due to them), unless they remit it or (the mans half) is remitted by him in whose hands is the marriage tie; and the remission (of the mans half) is the nearest to righteousness. And do not forget Liberality between yourselves. For Allah sees well all that ye do. [Quran 2:237]