Assume it is a PA. Several things must be done now. First, she must go NC with the OM, including leaving her job. Secondly, you must contact the OM's wife and tell her what is going on. Third, she must give you a written, clear, detailed timeline of the affair to be verified by a polygraph. Fourth, go see a lawyer to assess your options. Fifth, she no longer has any privacy concerning electronic devices and social media accounts.
If she balks at these requirements, file for divorce. Tell her that you will stop the process if she shows a true desire to save the marriage and the only way to begin to show it is to go NC, leave her job, provide a timeline, a give up all privacy regarding phones, computers, etc.
Don't be a pushover. If she doesn't want to comply, your marriage is dead and she is not coming back, nor should you want her back. You would most likely be her plan B. BUT first, notify the POSOM's wife of what has been going on. This is the best way to stop her little side fun.
But, most importantly, search your soul these next few months. Is this someone you want to be with for the rest of your life? She has thrown you and the marriage under the bus. She doesn't seem to love you anymore. Think long-term. It's hard right now, but eventually, you must come to grips with what she has done and how she feels toward you. Office romances seem to be the prevalent source of infidelity. Her situation is nothing special. She allowed it to happen. Don't stand for it. You are better than that.
