I never noticed this conspiracy until Saturday as I was reading this chapter (or at least I don't think I have), how the girls manipulated the entire thing, no doubt, as you indicated, orchestrated by Louisa with a personal agenda to accomplish. She was definitely on a mission, no holds barred.

]Why did they tell her? Did they maybe think they could proactively avoid her taking offense later? Or some other reason?

I don't think the girls have anything against Mary personally but they are young, lively and love to enjoy life. Mary's constant whining and complaining is at odds with this. She must seem 'such a drag' to these two young people. Not only this, they were keen to avoid her presence since Winthrop was the goal and she looked down on the Hayters. I imagine in the past any walks with Mary were full of complaints about, among other things, that she was tired, wanted to turn back, only liked short a walk, "long walks make me ill"... The girls seemed to be stopping by only as a quick courtesy call. Knowing that a short stop would produce curiosity by Mary, they probably thought they'd be safe by telling her they were going for a long walk. I can just hear them working this out before they left or on the way over. 'If we say long