But he's not guilty of aiding the enemy, only of offering to aid the enemy.
Sentences for conspiracies and offers to commit crimes are properly more lenient than for the actual commission of an overt criminal act (since offers may be repented of, and conspiracies undone before they come to fruition).
15 years in the slammer, and coming out as a decrepit, impoverished old man, is plenty, should it be proved that he was indeed guilty, and not the victim of entrapment by overzealous agents of the government.
He will never live through the 15 year sentence.