The beauty of this, btw, is that THEY DON'T KNOW WHAT YOU KNOW. Immediately, her mind will race to EVERYTHING she has done with him that is inappropriate, and wonder just which things you know about. Chances are that the photos from the bar and the formal event are but two of what probably are their MOST innocuous rendezvous.
I did this, with my wife. All I knew about was her text messages and some time spent in OM's car in a parking lot, but my voice intel then revealed them saying "I mean, I don't know what he thinks he knows. Unless he had a camera in the bushes that night at Jeremy's house outside the bedroom, how could he know anything?"