I am not sure, i've been asked this question by several people including my MIL. What do I do? Do I kick her out? I've already told her she should get a job ASAP and move out. Do I file for divorce? Should I tell her that I will not tolerate her continuing the A and communicating with him while living in my/our house? How do I enforce that? What are my options? This is all so foreign to me.
Find the link on Boundaries and study the concept. Then decide what it will take to protect yourself from disrespect. What will you do if she shows disrespect in your own home? Decide what you will not tolerate while living with another person in your house. Lies, deception, slander, stealing, show disrespect to you, etc. Would you allow a renter to stay under your roof who would do these things to you personally? How would you deal with it? Those are things you need to know before setting personal boundaries.
It is not about what you feel should work in your M. It is about doing the work that gets the right results. Do what works!