Spying being the operative word, it's very easy e.g. to plant listening devices, or drugs, or threaten with blackmail...

Being around him in an intimate way is gonna leak lots of stuff that’s useful for others.

I dunno if you’ve ever looked at your laptop in a hotel room with a partner around, but it’s pretty leaky.

And you might say that he’s a smart guy, he would be careful, he’s too canny to leak stuff. But then remember that he’s having an affair with a major customer and another with a direct report. He has already thrown caution to the wind.