Alastor let Devon settle herself against him. Through their various patrols the only threat within the castle at the moment seemed to be the students themselves. Losing his arm around her was a risk that he could afford to make. Plus should he need to awaken her quickly, even the act of getting up would immediately wake her up. It was practical. The fact that it was warm and that he did not really mind Devon leaning against him was secondary. After all he was a professional.
"Are you giving me orders now?" he teased lightly, but dropping his voice lower as it seemed she was settling in to actually fall asleep. "But yes. I'll get my thirty minutes after you. Go to sleep, Proudfoot. That's an order."