What is the interpretation of the voice a woman heard in the mosque, asking her to tell the questioner's wife that her husband is good and that she should not listen to people's talk, especially since the wife's mother hates the questioner and treats him poorly?

It is not permissible to be definitive about this matter based on conjecture. We ask Allah that it be for the best, especially since it occurred in a pure place. If what was mentioned about the wife's mother is true, then we advise patience with her and to counsel her with wisdom and good admonition from someone whose words she is likely to heed, along with supplicating for her repentance and guidance. The wife, in particular, must be patient with her mother's harm and beware of disobedience, for a parent has the right to good treatment, even if they err.