In his opinion, he was a perfectly safe person to be silly around. He certainly wasn't going to judge her for it, not when he was acting any better than she was. And there was nothing wrong with being silly and juvenile. Maybe many of the Slytherins believed otherwise, but that was there loss; life was so much more enjoyable if you weren't always so uptight and serious about everything.
"Yeah, sure," he responded, nodding. "I can always do with the extra warmth, all the better if I can potentially learn a new spell. You know what you're doing though, right?" he added. She had mentioned that she had been just testing out a new spell, and the last thing he wanted was for something to happen to him or his scarf.