Short answer
They should respect their parents while calmly discussing whether the objection is religious, practical or simply based on social custom.
Islamic Guidance
What Islam teaches
Islam commands kindness toward parents while also requiring people to follow what is right. Family consultation should be respectful and should not be turned into humiliation or emotional coercion.
Practical Guidance
What you can consider
Do not turn the matter into a confrontation. Present the facts, involve sensible elders and seek qualified Islamic guidance where necessary.
Sources
Qur'an 17:23; Qur'an 31:15.