When you are back at the house, Vernon asks for some privacy to help Omid. You need to find Clementine to check that she is ok.
You need to speak to both Molly and Ben. One of them will say that she is outside.
Molly is in the beroom upstairs. You can get to it through the bathroom or it is the door further along the corridor to the left of the bathroom.
Ben is downstairs with Kenny.
When you go outside, you will get the achievement.