Hi, I'd like to help you with your question.
For a marriage to be successful, honesty and trust have to be the foundation. And anytime someone has a relationship outside of the marriage, whether it is intimate or not, then that relationship undermines the trust of the marriage.
If your marriage is to be repaired, it is important that you and your wife talk this situation out. She needs to realize that the relationship she had was wrong and be willing to work on rebuilding your trust. She needs to be open with you and be willing to let you know where she is and who she is with. As your wife works on trust with you, you need to allow her to make up for what she did and work on eventually trusting her again. The marriage counselor should be able to help guide you both on how to rebuild your marriage.
You can also work on this problem at home. Here are some resources to help you get started:
Not "Just Friends": Rebuilding Trust and Recovering Your Sanity After Infidelity by Shirley P. Glass and Jean Coppock Staeheli
Intimacy After Infidelity: How to Rebuild & Affair-proof Your Marriage by Steven D., Ph.D. Solomon and Lorie J., Ph.D. Teagno
Unfaithful: Hope and Healing After Infidelity by Gary Shriver and Mona Shriver
You can find these books on Amazon.com or your local library may have them for you.
I hope this has helped you,