If anyone is crass enough to demand an explanation for why Awful Woman was not invited to your birthday party, or is wondering why you are absent from the BBQ while you are in town, I think your mother should tell them the truth: "Awful Woman does not like GSNW, so I did not invite her to the birthday party. Beandip?" or "Awful Woman does not like GSNW, and has shown so in her mistreatment of her, but it was more important to me to have Awful Woman at my BBQ than GSNW. Beandip?" (This is if you're comfortable with that.)
I think that's a lot of drama, and I think it's unfair to the OTHER people to give them that answer.
Even if they did pry, they shouldn't be dragged into something.
And it is not *automatically* prying to say, "Oh, could Awful Woman not come to the B'day BBQ? I know GSNW and her daughter were very close! I thought she'd be here."
Nor is it automatically prying to say, "Oh, I was sure we'd see GSNW at the big BBQ, since she's visiting you now. Where is she?"
And GSNW's mother is the one who has to live with the drama that might get created.
Why create it? GSNW didn't really ask for that--she just wants not to deal with this woman. Bringing up the antipathy is just going to make GSNW and her mom look bad. AND it's going to mean that the meanness of Awful Woman has a far longer life and impacts even MORE people.
I'm not saying GSNW needs to absorb more abuse. Far from it. In fact, I think creating drama will draw MORE abuse, because every time she comes home, all the other people will try to argue her out of feeling abused, etc.
I'm just saying, "Let it drop, as a topic of conversation. Let it go away. Don't bring it up." And don't invite her to the party.
The idea behind cutting someone off is that you don't keep giving them headspace. Or conversation space.
(oh, yeah, and happy b'day, GSNW!)