Is a mother's intervention to reconcile a dispute between non-Muslim neighbors considered among the rewards for reconciling disputes, especially since her husband sometimes asks her to do so?

Reconciliation between people is a righteous and religiously commendable act. This includes Muslims and non-Muslims, especially if they are neighbors, for a neighbor has a confirmed right, even if they are non-Muslim.

The basis for reconciliation between people is the Almighty's saying: (There is no good in much of their private conversation, except for whoever enjoins charity or what is right or reconciliation between people. And whoever does that seeking means to the approval of Allah - then We will give him a great reward.) An-Nisa/114.

Also, what is mentioned in the Hadith that every mediation between two people is charity, as stated in Sahih Muslim. This includes reconciliation between Muslims and non-Muslims. The evidence for this is that a neighbor has a confirmed right, even if they are disbelievers; based on the Almighty’s saying: (and the neighbor who is near and the neighbor who is a stranger) An-Nisa/36.

Scholars have clarified that reconciliation is permissible between Muslims and non-Muslims, and that the mention of Muslims in some texts comes by way of predominance, as they are primarily addressed by religious rulings.

Therefore, there is no harm for your mother in reconciling between the disputing spouses, even if they are disbelievers, and she will have a reward, Allah willing.