So I currently work with two of my friends. We'll call them Jim and Lisa. I've known and worked closely with Jim for over a year. I've known and worked closely with Lisa for 6 months. Jim and Lisa are in a relationship and expecting a baby. Jim confided in me that he is cheating on Lisa with another of our coworkers. Jim has cheated in the past and Lisa is aware and not really trusting of him, but Lisa wants to make the relationship work and wants to trust Jim. Lisa is hesitant about Jim working with the coworker that he is in fact cheating with. So my question is do I tell Lisa, or pretend like I don't know for the sake of maintaining the friendship/work relationship? Do I pretend I don't know and just let it go? Also, bear in mind, it seems that a lot of other people at work know about Jim and the other coworker.