Why not tell her? She will find out eventually, there are many who know the two of you and it will not take long for someone to make the connection.

[He stiffens somewhat at the mention of her secret, you know the one he kept for the most part.]

It was a secret. I only told you where I was, not the circumstances. And I did not speak a word of it to anyone else. [Then it dawns on him.]...She believes I told you, doesn't she?