Learn Russian: Он говорил с мадам Дюпон весь вечер. - He talked with Madame Dupont all evening.

Foreign titles like мадам and мисс never decline in Russian. Additionally, foreign surnames ending in a consonant, like Дюпон, are indeclinable when they refer to a woman. If he had been talking to a man named Dupont, the surname would decline like a standard masculine noun: с месье Дюпоном.

If the woman's foreign surname ended in a vowel instead of a consonant, would it decline?

It depends on the vowel. If a foreign surname ends in an unstressed -а or -я, it usually declines for both men and women. But if it ends in a stressed -а (like Dumas) or other vowels like -о, -и, or -у, it stays indeclinable. Since Дюпон ends in a consonant, the rule is strictly based on her gender (indeclinable for women, declinable for men).

Why is весь вечер in the accusative case here?

The accusative case is used without a preposition to express the total duration of an action, answering the question 'how long?'. Because he talked for the entire evening, it takes the accusative. If you just wanted to say that the conversation happened 'in the evening' as a point in time, you would use the instrumental case: вечером.

Why use the imperfective verb говорил instead of a perfective verb like поговорил?