She looked so sad now and he couldn't just let her stand there, being upset. He was still horrified and he was going to have such a rant later to whatever friend would listen to him. But this was his sister, and she was going to be in whatever relationship she wanted. He didn't have the right to tell her who to be with, or not to be with. He could just be there for the fallout.
"Oh, hey- No. Oh, Arya, I'm sorry," he said, holding his arms out for her. "You're right, it won't be like that forever." One way or another. "It's alright."