Is the OM married? If so, I would expose it to his W. If not, there may not be much you can do. I see many people here that allow their WW to remain in the house while an A is ongoing. You may not be able to legally kick her out but you can try. At a bare minimum, kick her out of the bedroom. I wish to God that I had done more when I first found out. I would be very heavy handed. I know first hand that being nice and showing compassion does not work. They will lie and manipulate to get what they want. Being lenient does not work. Trust me.
Me: 48 y/o W: 47 y/o Together: > 20 yrs BD: Dec '15, then S 2nd BD: Mar '16, then I filed for D April '16: started piecing