"A risk for which one of us?" Corwin asks. He needs that clarification before he can know how to properly argue against it-- or if he even should. If staying with him is a risk to her, he certainly doesn't want her to take it if she doesn't have to. But he can't see how it possibly could be a risk for her, that's the whole point of the offer, because he can only see how it would be safer for her than staying out all day, alone.