If I recall correctly, this would be an illegal order in violation of the Title 10 authorities which govern DoD activities if it took place within the borders of the country. I'm not a federal lawyer so I don't know the details, but I believe it prohibits the armed forces operating under Title 10 from performing operations on U.S. soil. Title 18 however governs agencies like the FBI and allows operations within U.S. borders but prohibits foreign operations.

This is all a vague memory from an old lesson, but at first glance the attorney's argument is utter crap. It boils down to "no act of the president can be considered in violation of any law." Not a comforting sentiment, and a dangerous precedent.