Short answer
They should separate the person from the disagreement, listen carefully and seek objective advice rather than using insults or emotional pressure.
Islamic Guidance
What Islam teaches
Islam teaches respectful speech, kindness to parents and justice. Disagreement does not justify disrespect or abuse.
Practical Guidance
What you can consider
Limit unnecessary involvement, write down genuine concerns and use one or two trusted mediators instead of allowing the matter to become a family-wide argument.
Sources
Qur'an 17:23; Qur'an 49:11-12.