No legal way around it...as I understand it, changes to the floorplan are likely to change the assessment value. If you get a building permit, it will be noted, and most likely assessed. If you don't and they find out, it is a fine, then they back assess and you must still get a permit to have the work checked, which may require tearing some stuff out.
Your friend may be on borrowed time if they ever do a drive-by looking for unpermitted work. This would be even more evident if the garage door was removed.