I believe she is a Royal Highness now, however -- ever since her marriage in 2008 to the Duke of Apulia. Correct?
In this, her situation would be comparable to that of HH Princess Alexandra of Fife (who became a duchess in her own right). By all accounts, her uncle (King George V of Great Britain) was extremely unhappy that she and her sister were granted princess titles in the UK, with the qualification of Highness. But there was nothing he could do about it, since they had been conferred upon the girls by their grandfather, King Edward VII, after he ascended the throne. As sovereign, he was the fount of all honors, and the son had to respect his father's actions.
That being said, I'm assuming that George V later on, as king, did approve as dynastic the marriage in 1914 of his niece to his cousin, Prince Arthur of Connaught -- correct? Also, he could not object to Alexandra henceforward being styled as a Royal Highness (as this presumably was in accordance with English common law).