Scout assumes that Mrs. Merriweather is talking about Mayella because she considers Mayella to be the one to have sinned, not Mrs. Robinson.
Mrs. Merriweather is described as “the most devout lady in Maycomb” (ch 24). Since Scout is sitting next to her in Aunt Alexandra’s Missionary Circle, she feels it is only polite to strike up a conversation with her. Mrs. Merriweather is concerned with the children in Africa, but seems to have no interest in the people of Maycomb who also live in squalid conditions.
Mrs. Merriweather explains to Scout that she should be grateful.
You live in a Christian home with Christian folks in a Christian town. Out there in J. Grimes Everett's land there's nothing but sin and squalor. (ch 24)
When Gertrude, sitting beside Mrs. Merriweather, mentions something, Mrs. Merriweather mentions “forgive and forget.”
Thing that church ought to do is help her lead a Christian life for those children from here on out. Some of the men ought to go out there and tell that preacher to encourage her. (ch 24)
Scout assumes they are talking about Mayella because in her eyes, Mayella is the unchristian one. Mayella tempted Tom, cried rape, and got him killed. Scout does not consider Tom Robinson’s wife to ever have acted in an unchristian way, so she would not think they were talking about her.
The Missionary Circle ladies’ conversation demonstrates the hypocrisy of these women. They worry about poverty in Africa, but care nothing about the treatment of blacks in their own town. Even after everything learned at the trial, they still blame Tom Robinson and his wife. They care nothing about helping them, only say that someone else should.