"I changed the password every 3 days and never memorized it. Current password was on a post-it on my monitor. Did you guys lose the post-it?"
Simple as that, right? They can't compel you to remember information you never had in memory. It's probably too late, as he's likely admitted to remembering the password. Dumb move.
Polygraphs are pseudoscience. The judge /might/ rule a polygraph as admissible evidence -- most will not, because of its inaccuracies -- but a "lie detector" is a gimmick.
Simple as that, right? They can't compel you to remember information you never had in memory. It's probably too late, as he's likely admitted to remembering the password. Dumb move.