Learn Russian: Ты сдал экзамен? Молодец!

In Russian, the imperfective verb сдавать refers to the process of 'taking' an exam. The perfective verb сдал, which is used here, emphasizes the successful result—meaning 'passed'. If someone took the exam but failed, they would say Я не сдал экзамен.

If I am speaking to a woman, do I need to change Молодец to a feminine form?

No, молодец remains exactly the same. Even though it is a grammatically masculine noun, it is used as a 'common gender' word to praise anyone—a man, a woman, or a child.

Since this lesson is about the truncated vocative, how would I use it to address a friend named 'Sasha' in this sentence?

You would drop the final vowel of the name to get their attention informally: Саш, ты сдал экзамен? This short, colloquial vocative form is very common among close friends and family for names ending in -а or -я.

Why is экзамен in the nominative case here if it receives the action of the verb?

It looks like the nominative, but it is actually in the accusative case because it is the direct object of the verb. As a quick refresher: for inanimate masculine nouns like экзамен, the accusative form is identical to the nominative.

How would this sentence change if I were addressing a group of friends who just finished their test?