He’s on a battlefield in enemy held territory providing support to the enemy. We don’t need an excuse to kill him. It doesn’t matter if he is a US citizen. Think of it this way - Lord Haha and Tokyo Rose were, respectively, British and American. But the provide enemy propaganda targeted at our citizens and troops. We would not need legal review kill them. This is the same case here.
But “...the United States can kill its own citizens without judicial review when litigation would reveal state secrets” is an effin’ absurd argument. I’d have that guy fired so bloody fast.
I wouldn't be surprised if the lawyer was an Obama holdover deliberately trying to throw the case: that's what one of the officials in DHS did with DACA. She so weakly defend Trump's position that his authority to end the program was postponed.