She not only survived the descent of Elturel, but made it out with a Soul Coin. She is first seen during the cutscene at the end of the Prologue, peering through a telescope.
Nadira's mother was a warlock, which she credits with teaching her to see through fiends and their servants[1]. Despite this, their relationship was rocky enough that Nadira ran away from home at sixteen[2].
Nadira can be found above the entrance to the Grove. She is being stalked by a Bugbear Assassin. Should the party decide to help her, they'll fight the bugbear together. Talking to her after the fight will also reward 35 Experience.
Despite being saved from the assassin, she will accuse the party of being sent to retrieve the Soul Coin because of having the scent of Avernus on them. After passing a Deception, Persuasion or IntimidationCheck at a DC 5 with a situational Advantage, she will either thankfully or bitterly (depending on the skill check used) reward the party with the coin.
Nadira's survival is Time Sensitive. If they party gets too close to her location but doesn't help her, the Bugbear Assassin will murder her.