This won't happen overnight. It takes a LONG time to recover from the betrayal of an affair. Because your OP was his best friend, he was served a double helping of pain. What you can do is to be completely open and honest with him about everything from here on out, you can be accountable for your time and actions, you can offer complete transparency (give him passwords to all emails, cell phone, etc).

Have you written a NC letter to the OP and let HIM send it? Are they still friends? As long as there is ANY contact with OP, you are back to square one. Read the articles here. You may even want to call the Harleys for some one-on-one counseling. Your fiance is in for a long journey of pain. Will he come here and post?

Widowed 11/10/12 after 35 years of marriage
In a sense now, I am homeless. For the home, the place of refuge, solitude, love-where my husband lived-no longer exists. Joyce Carolyn Oates, A Widow's Story