Ask your spouse if he or she is willing to end the other relationship for good and cut off all contact. I'm not just talking about physical contact. For the marriage to recover, there must be no contact, electronic or otherwise. This other person must be completely out of the picture; it will therefore be necessary to un-friend this person on Facebook, un-follow this person on Twitter and delete this person from all phones. If your spouse works with this person, collaborate with the couples therapist to set up the parameters to manage this difficult ongoing challenge. Perhaps your spouse can change divisions or departments or at least cut out the after-work office happy-hours.
Is your spouse willing to cut off contact? Is your spouse willing to let you know when this person attempts to make contact (which they will) and to discuss with you whether and how to respond? If your spouse is willing to completely cut this person off, that is an excellent sign. If you are hearing all kinds of excuses, this is cause for serious concern.
Unfortunately, your spouse's infidelity is something you never forget. However, this does not mean that your relationship is doomed. The steps listed above are by no means easy, but if your spouse is willing to do whatever it takes, then staying together is a choice worth considering, especially if children are involved. Remember that infidelity is much more common and much more complicated than people realize, and you are not alone in this difficult journey.