If you've ever personally dealt with a pathological narcissist, you'll know that there is really no way to remind them that they are delusional that would be perceived by them as "polite." The key to understanding narcissistic identity is that they experience any challenge to that identity as an attack on their very existence; it's as if you are literally trying to murder them. It causes them that much distress. It's one of the reasons they are so angry all the time. Reality already chews at them all day long.