What is the appropriate action for someone who receives unseen predictions about their life and future, and is the one who makes such claims sinful for claiming knowledge of the unseen, and what is the best way to deal with them and correct their misconception?

Knowledge of the Unseen is something Allah has kept exclusively for Himself, and none knows it but He, Glory be to Him. What your brother mentioned in his message is a form of soothsaying and claiming knowledge of the Unseen. You should advise him to repent and explain the seriousness of his action. You may even forsake him if that would benefit his repentance. His quoting of the verse, "He taught man what he did not know" is a flawed inference, because the verse refers to acquired knowledge, not knowledge of the Unseen.