If she is still under foot working at his office most likely the affair has gone underground.
Watch him like a hawk and, psst (place a voice activated recording device in his office.) Go in a week later and retrieve it, hide it well.
One dirty deed deserves another.
I am gay so maybe I am not the type of man's advice you were seeking. lol
Don't blame me for the results.
A friend of mine hid a recorder on his girlfriend and found out she was having an affair with another woman.
She was so angry with him she broke up with him.