What is your FIL's position in all of this? He appears to be silent on the whole matter and her mother is a spineless enabler at heart...

AT least she is TRYING to be diplomatic... But man, what a putz

How about if you ask her mother to tell your wife that OM will NEVER be welcome in their home and will never be accepted by her or FIL as a member of the famly.. EVER

Would MIL be willing to say THAT to your wife?