You don't allow it to continue under your nose. That's what exposure is all about. Additionally, plan A means you're supposed to let your WS suffer their own consequences of the affair.

Therefore, you continue to show him how much his continued affair hurts you. You continue to expose whenever he contacts OW, you keep the pressure on him for NC to take place...and at the same time keep up the carrot of plan A...make yourself a better choice over OW, by meeting his EN's where you can and making positive changes for yourself.

Make sense?