She doesn't trust people's words as far as she can throw said people, not unless she has some kind of leverage against them not standing by it. But this might be something, in hopes that keeping this a secret is even half as important to him as to her, and she'll take the chance.

"Only if you swear the same to me." And, just to close loopholes - this one takes her a moment to gather: "And also that you won't rely any information that you learned in it to someone else."