From the information you provided, I am of opinion that she is bound to your cancellation policy as communicated to her, being that if you cancel, then the deposit is non-refundable.
Unless you have committed a serious breach of the agreement, she has no legal ground for the cancellation and therefore, no legal ground to claim back her deposit This does not seem to be the case, though.
The Consumer Protection Act will also not assist her. The Act specifically excludes special order goods from a consumer's right to cancel pre-ordered goods.
I am of opinion that you can, therefore, tell her with confidence that she can take whatever legal action she wants, that you will complete the dress for her if she wants, provided she pays the balance before delivery and that, if that is not acceptable to her, you are entitled in terms of your cancellation policy to retain the deposit.
You can tell her that you will defend any legal action that she may bring against you and you will hold her liable for your legal costs.
I hope this helps you a little bit, but if you have any further questions, feel free to ask them before you rate. It will not cost you anything extra. If you are satisfied with the answer, kindly click on one of the five faces on the ratings page. I don't get anything for this answer unless you do.