How can one help a young man whose mother was insulted by neighbors, and he regrets not defending her honor?

Harming and insulting one's neighbor is a characteristic of the Age of Ignorance (Jahiliyyah). The texts of the Qur’an and Sunnah (definition) have successively enjoined good treatment of neighbors and kindness towards them, and condemnation of this (harming neighbors) is a great sin.

The most important advice that should be given to a Muslim is patience and to counter their ill-treatment with kindness, so that perhaps animosity may turn into affection. Allah Almighty says: "Nor can goodness and evil be equal. Repel (evil) with what is better: then will he between whom and thee was hatred become as it were thy friend and intimate!" (Fussilat: 34). Advice should be offered to the neighbors, and if their situation does not improve, then one should file a complaint with the responsible authorities. Otherwise, one should seek to move from the place, for Allah’s earth is vast.