Is what the girl did – telling someone she had died to get rid of his harassment after an illicit phone relationship – forbidden, and will her repentance be accepted?

You should have been firmer with the foreign man, and threatened to report him to someone who could deter him, instead of lying. You could have used tawriya (ambiguity) or ma'arid (indirect expressions) by using the word "death" to mean sleep. Since the lie has already occurred, we hope there is no sin upon you if it was necessary for an overriding benefit. One who repents from a sin is like one who has no sin. Therefore, be diligent in avoiding men and not engaging in illicit relationships. Occupy yourself with beneficial knowledge, memorizing the Quran, and the company of righteous women.