I guess you could remind the mil of this..."possible affair, drug use, lies, manipulation, emotional, verbal and physical abuse...." and that he chose to act the way he did and to remind her of those facts.
Why should you suffer more just to placate him!!???
Sorry you have having this pressure from the mom-inlaw. Why is your friend on the RO as well?
My exabf's parents had dealt with his 2 older brothers having addictions, so they always told me that I was enabling him by letting him stick around...even though they didn't put it in those words.
maybe you could give them something to read about addiction and enabling so that they will 'get it'?