Previous versions of the game have had issues with this achievement, but as of april 2018 this is what worked for me.
1. Talk to everyone in the settlement after the tutorial (Daina, her husband, her child Erin)
2. Go get the first key
3. Go back to the settlement and talk to everyone again
4. Clear the first dungeon
5. Once more, go back to the settlement and talk to everyone (not sure if this matters but your mentor should be in the cave at this point, so feel free to talk to him, too)
6. Collect the second key.
7. Go back to the settlement and talk to everyone (beginning to sense a pattern here?) By this point Erin should be hiding behind a bush on the bluff overlooking the camp, near they sky boat.
8. Clear the second dungeon.
9. Go back to the settlement. At this point Erin's dad should mention that she's missing and ask you to check the outskirts of the island. Erin will be to the northwest between a tree and a spar of dark rock. An easy way to locate her is to exit the cave your mentor is in by the northern entrance and just ahead and slightly to the left is a small rise of hill that has the tree and rock Erin is standing between, though she may not be easily visible until you get closer.
10. Talk to Erin and the achievement should pop. (You can also tell her dad you found her, if you're feeling magnanimous, though it's not required)