After a short cut-scene, you get new directives. You have to get close to Erin without being detected. Start from finding the highlighted area where Erin is but don't approach her. Use the side and upper passages instead (you can get to them with a claw) to find yourself behind girl's back.
Crouch and hold also the button for slow moving and approach Erin. Pressing the interaction button makes her flee to the chapel again. If you make a mistake Erin will teleport to the other place. It's not a problem, because Garrett won't lose anything in that case. Plan your route from the beginning and try to sneak upon her once again.