There's no flash of surprise in his expression at the admission she gives him, which in itself is quite telling; he already knows where she came by the idea. This conversation is purely for the purposes of confirmation. Of course, he can't help but be a bit tempted to spring other pieces of knowledge on her - but that's fun and games for a later time.

"Presumably, you've advanced in your education enough to recreate the spell for that purpose, then." He says it dismissively, but there's a slight intonation which suggests that this, unlike his other comments, is a question. Could she, if necessary, create a Dark Mark. Or something similar.