T.S.
There are a couple of things here that make me think you may be being used in a situation you don't really know enough about. First of all, why is it YOU that is being asked to contact this woman? Why not the father himself, or your husband or SOMEONE who was EVER related to her.
I have to say that the fact that she " kept moving further away and getting judges to side with her in imposing strict visitation guidlelines" points to there being a bit more to the story than you may understand. Family court judges do NOT create obstacles to parental relationships, they protect them, so if there are strict guidelines, they are there for a reason... also, guidelines or not, why isn't he seeing his kids?
That said:
My advice is to send her one last email in support. Tell her you're sorry for your part in all of this and for not being there for the kids. Apologize for whatever you have to apologize for and tell her the truth about the grandmother and let her do with that what she will. YOU may think it is mean to the grandmother if she can't see the children, but SHE is responsible for the CHILDREN. Your mother-in-law wants to see her grandchidlren for HERSELF, not because she thinks THEY want to see her. Give their mother the information and she can decide what is best for her children.
HTH
T.