The reply of the philosopher was that no man must be called happy until he has ended his life in a fitting manner.

In indirect speech, the reported clause's tense is changed according to the tense of the reporting verb. Here, the reporting verb is in past tense but the sentence is a moral principle that can be considered a a universal truth, so the tense will not change. "Call" here refers that it is an imperative sentence, so "must" is added here.