I agree. In addition it might even save his job. If the 30 day period is a chance for employees to improve the manager might not find any reason for firing him

I agree. In addition it might even save his job. If the 30 day period is a chance for employees to improve the manager might not find any reason for firing him if he performs really well. You also show your evil manager and everyone else what a professional you are, even if he isn't. I got fired once after 3 months at a company, without any warning or hints of my boss not being satisfied with my performance. I was completely devastated but decided to double down on my efforts for the period I had left. A week later my boss called me to his office and said that he had made a mistake firing me and offered me my job back. (I didn't accept)