You'll need to head towards Nekarow (3 to the left, 2 down from Nest). Jump down from the top section to a slightly lower section with a small creek becoming a huge waterfall. Jump down with the waterfall, you should be in a little spot of land between two falls.
Not sure if order matters, but I entered the left (if you were looking at them) first and Kay should mention searching behind them for secrets). Now check the right fall and when Kay shouts for Yu, have her replay "I'm coming" (not sure if choice ultimately matters?).
Achievement should pop shortly after.