Did you speak to her in the city, or in the map directly above the city? Then there's another cutscene in which Arianna jumps out of the bushes, scares the MC and ‘complains’ about being lonely and would like to play hide-and-seek in the Darkwood with the MC again, like she used to as a child. I think only then is it triggered that she can be found near the ruins. But I'm not quite sure, it was sooooo long ago.
Anyway, if everything goes right, she should be here during the day (at the beginning of the story).
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