She is about to answer his question when he adds the part about a human sharing his face, which is just confusing. She pauses, hesitates, and then decides to just go with answering the first question.

"Much of magic is based on sympathetic connections... joining things with relationships, either emotional or physical. It's important to be able to guide the magic to do what you need, and the best way to do that is to provide it with a path to follow.

"And so... theoretically... if a new arrival still had a strong sympathetic connection to the realms outside of this one, it could be powerful enough to bridge the gap. That, combined with the fact that the ward is already damaged from excessive use."