This one is pretty simple. When you first get into the park and move through to the Fairytale Town area, you will find Arthur lying down leaning against a tipped over barrel. Speak to him, give him a heal to help him recover and then Mara will help him to her car as a black screen comes up. Once cleared, he will be gone. Simply run back out of the park the way you came and you'll find Arthur in your car. Talk to him and the achievement will pop!
Simple!