Which is being a very good friend to somebody who is being a very bad friend, which is the definition of being a good friend. People don't need us when they're feeling great, and they don't try to hurt us when they're feeling accepted. What Eleanor did was make her a spectacle -- "fun" or not, "harmless" or not -- which means somewhere in Eleanor's head she's looking at Jackie's actions through a magnifying lens, which is the one thing Jackie can't allow. By taking the scattered moments of Jackie's life and applying a narrative to them, she's given Jackie a cohesive story through which to view her own actions: a story which directly contradicts her self-image as a hardworking mother and wife, while underscoring her secondary self-image as a piece of shit. Which, if you care about the person as much as Jackie does Eleanor, is essentially like Eleanor calling her a piece of shit.
But since this is all on the level just beneath the skin, and because Jackie lives on top of the skin, always, it's like Eleanor called her a piece of shit behind her back, inside her head, under the skin. And there's a level on which that's true, but it's much more true that Eleanor couldn't have known she was stepping on any landmines, because they've always connected about how they both live on the skin, on the surface of things, so a conversation about how little Jackie's adultery signifies was supposed to be more glue on their relationship, not this huge fucking problem. Anything can be a weapon when the world turns dangerous.
And when you realize that you are dealing with somebody's demon, which she is accidentally but now obviously doing, all you can do is step back one giant step back and say Mother May I, because you have to demonstrate that your love is bigger than their fear. Not "I love you to the diameter of this thing that is hurting you," but just a little wider than that. You use your love to make the world big enough for them to move around again, and not feel so caged up. Hopefully they come back to you, maybe they don't, but either way the problem already happened, and neither of you were prepared, so you might as well be good, and remind them where home is.