Your MIL is free to keep in touch with, friend and talk about anyone she chooses! What do you really think people are going to suggest? Forbid her from caring about an ex-daughter in law? Have your husband YELL at his mother & deliver an ultimatum?
If you're as classy as you say--you'll handle this with some grace and dignity!
YOU need to change the way you react to her actions.
A side note.....my mother & father were divorced for YEARS, my mom and her MIL (though, technically "ex MIL") stayed close, for the sake of the kids and family.....after my father was dead, my mom continued to see her MIL, care for her, take her to doctor's appts, etc. My mom was the executrix of her estate when she died. My mother and HER ex MIL understood that they were two people that cared about each other, regardless of labels.
Just because there may be "bad blood" between a man & ex wife, the classy thing is NOT to lump all of the ex husband's relatives in the same camp. Sounds like your husband's ex and his mom "get" this. Hopefully you can too!