Lies and evil thrive in the darkness. They need to have light shone on them to dissipate.
Look, you know the truth, and OMs spouse strongly suspects. This is a somewhat different sitch than if she suspected nothing. She has asked you point blank. If you don't tell her what you know you are aiding and abetting the adulterers. There are lies of omission as well as lies of commission.
JMHO.
H53/W51, R-ing 4/'18
"Do not arouse or awaken love until it so desires"-Sg.of Sg 2:7
"So oftentimes it happens,that we live our lives in chains, & we never even know we have the key"-Eagles III 1:3