How to disempower: back away and don't be friends, and tell them why!
Flat out say "I don't trust your judgement about ___because you are so impaired about ____."
Call it denial to their faces.
Refuse to talk about____ with them: and tell them it's because it's useless.
Urge them to get mental health help for denial.
Don't give them perks or promotions.
...I know it's not much! But unfortunately we do not have a lot of good ways to extract people from it (I'm a mh professional)