There is some visible conflict in Ellis' expression. On the one hand, that would be very useful. On the other, why does it have to be done in the house?

After a moment: "You're terribly clever, you know."

Because that's the truth of it, regardless of Ellis' apprehensions. The entire matter can be distilled to that single truth: Wysteria saw opportunity and pursued it. She is blindingly clever, and he is proud of her for it.

"But be sure you mind you don't do yourself harm in the process, yes?"