Arette had managed to convince Dominic to let her go back to Evermore, partly because Grace had said she had something important to tell her. Arette was eager to find out what it was, and also because she was feeling bored and frustrated being stuck in the mansion. She knew she was missing out on lectures and valuable lessons by staying there.
So, she had asked Dominic if she could return to Evermore, explaining her reasons for wanting to go back. He had initially been reluctant, but after some persuasion and reassurance from Arette that she would be careful and safe, he had reluctantly agreed.
Despite his hesitant agreement to let her return to Evermore, Dominic did insist on driving her there. He knew he could trust her, but he couldn't shake off a sense of unease about letting her go back to the academy. What if the environment triggers a memory that he doesn't want her remembering.