C.O.
I would leave them be. Divorce or no divorce - she is STILL part of your family as the mother of your grandchildren. Your grandchildren STILL come to YOUR HOME - just because their parents are divorced - does NOT mean THEY are divorced from her. You can't obliterate her from your life. Your grandchildren STILL need to see that you care for their mother even if your son and she couldn't make the marriage work.
If the new W. can't accept that - then she's not the right W. for your son.