Sarah's use of the style Duchess of York is the standard use available to a divorced wife of a peer and thus she is Sarah, Duchess of York, just as the divorced wife of Mr John Smith is Mrs Jane Smith.
If Sarah ever remarried - someone other than Andrew - she would take on her new husband's name and title (if he had one). Should that marriage also end in divorce she would be able to return to using the style Duchess of York but she isn't The Duchess of York - that title can only be held by the actual wife of The Duke of York.
Please could I try to help with a little background information?
Sarah is Sarah, Duchess of York; the UK media doesn't help by continuously calling her 'The Duchess of York'. The papers are also wrong when they call her 'Sarah THE Duchess of York'
As Iluvbertie correctly says, Sarah has lost her 'HRH' title, which happened on her divorce, when she also lost her precedence. [her 'rank' (if you can call it thus) is based on her late father's station, and so she is the younger daughter of a gentleman- some lists take into account the fact that her late father was an army officer, others simply refer to her father as a 'gentleman'.
If Sarah was to re-marry, as Iluvbertie says, she would take any style and title of her new husband. If she was then to divorce her new husband, in fact under British law her use of her former style of 'Sarah, Duchess of York' is not 'revived' in the true sense, as if it had 'lain dormant' for the duration of her subsequent marriage. Former wives who do 'revert' to their former titles are actually doing so because their 'entitlement' to do so is actually based on the English and Welsh legal right [I am not sure about Scottish legal position] for anyone to call themselves what they want,
with the proviso that by doing so you are not attempting anything fraudulent.
The most well-known examples I can think of are Raine, stepmother of Princess Diana, who, after the death of Earl Spencer then married the Comte de Chambrun. Raine soon divorced him, but instead of calling herself Raine, Comtesse de Chambrun, chose to revert to the Style 'Raine Countess Spencer'. She is doing so though based on the premise that you can call yourself what you like providing you are not being fraudulent.
Other examples of ladies who reverted to the style of their former husbands include Maureen, Marchioness of Dufferin and Ava, who, after the death of the Marquess, married Major Desmond Buchanan, but retained her 'title' "out of deference to the wishes of her first husband"
Hope this helps,
Alex
