Is a woman permitted to refrain from visiting neighbors who do not reciprocate her good treatment, and to refuse her husband’s request to visit them, and what is the sin or expiation incurred if she requests a divorce from her husband due to his insistence on helping them?

First, thank Allah for overcoming the ordeal, and thank the husband for his wisdom and the wife for not persisting in her demand for divorce. The husband's good treatment of the neighbors, despite the neighbor's wife's bad treatment, is the essence of wisdom and is in accordance with Islamic law. Demanding divorce merely because of this good treatment is impermissible and one must repent from it. You are not obligated to visit your neighbor, especially if it results in harm, but it is not permissible to abandon her. Maintaining ties with her and enduring her harm brings greater reward and is more conducive to a good relationship between you and your husband.