Is the idea of time travel disbelief (kufr), and is its perpetrator excused by ignorance in a society where sins are prevalent?

It is not permissible to rule your friend an infidel (kafir) for his mistaken belief and false statement, as this falls under the category of interpretation (ta'wil) that prevents excommunication (takfir). The falsehood of his statement is clear in the verse: "And no soul perceives what it will earn tomorrow, and no soul perceives in what land it will die." One should be mindful of the seriousness of speaking about matters of excommunication, for one whose Islam has been established with certainty does not lose it through doubt.