Now how the hell does he tell this sensibly, without lying, and without making her think he has gone thoroughly mad?

"I discovered that someone has been spying on me for some time. We exchanged words. I was not polite. It was reckless of me to be so cavalier without knowing my adversary. Among other things, this individual threatened to—to damage your father's sanity. You would know better than I whether such an attempt might be successful."