A Question About Guantanamo and The Court

I had a discussion with a coworker this morning and we got around to the Supreme Court hearing a case in which the authority of the President to hold prisoners without resort to our legal system. My coworker thought it was fine until I asked this question. If the court finds that the US legal system has no jurisdiction over prisioners held at Guantanamo, and if that decision is announced on some date, say the 4th of July, could the President then legally order the execution of every prisioner there on the 5th of July? That question marked a real change in his thinking. You might want to try it if you find yourself in a similar discussion.