Though I'm not overly enthused about Galadriel's "young captain of Gondor" lines, there is a possible explanation - her mirror might have shown her what was happening. However, the nature of the mirror being as it is, it's not certain that what is shown is actually happening in the present or will certainly happen in the future. Therefore, she would not intervene even if she were able to.
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'Mercy!' cried Gandalf. 'If the giving of information is to be the cure of your inquisitiveness, I shall spend all the rest of my days in answering you. What more do you want to know?' 'The whole history of Middle-earth.. .'
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