What is hard is if she is a member of the household, she does kind of have a right to be with whom she wishes in the home. While I don't agree with adultery and am not sure why your mother in law is making this choice, it IS her life. We are all allowed to make our own bad choices. I'd feel more sad for her than irritated. If you approach it with empathy--- as in, is she selling herself short to attach to a man who is married, would she want to change the situation? As opposed to feeling judged? What is the status of this man? Married and living with his wife? They are separated? They are in the process of divorcing? Is it culturally looked down upon to be divorced and is he staying with his wife because of this-- for her benefit so that she doesn't live with a stigma? These situations can be complex and if we try to approach it from the stand point of caring and loving something (your mother in law)-- maybe you can be helpful to her and alleviate the problem of a married bf by her own desire to have something better in her life. If you just tell her its wrong, she'll dig in. Ever hear of Romeo and Juliet? Good luck. If it is your home, then can ask to not live with inlaws but this may cause a marital problem with you and your spouse. Otherwise, you have to respect the rights of others to make bad choices. good luck
please give your mother in law some space if that guy is not causing any harm to you or your family...
Do you have children? It might help if you could say..
"Listen, I have been very understanding and have wholeheartedly accepted your mom into our home for you, however, I cannot live with her bringing a married man into our home. I know we cannot stop her from being with a married man, but we can let her know that we have morals and values, and believe it is sinful, and do not agree with what she is doing. We can tell her to go to a hotel if they want to be together. By saying this, it does not mean that we are not still supporting her, it merely says that we are not willing to throw your own morals out the window by enabling such wickedness as adultery."