Short answer
They should discuss relevant facts respectfully and avoid turning a person's previous marriage into gossip or humiliation.
Islamic Guidance
What Islam teaches
Islam prohibits backbiting and encourages justice. Understanding relevant circumstances is legitimate, but spreading private details unnecessarily is not.
Practical Guidance
What you can consider
Focus on information that genuinely affects future compatibility. Speak privately and respectfully, and avoid collecting or spreading unnecessary allegations.
Sources
Qur'an 49:12.