What would you do if a person you know to be mentally unstable said they passed a background check and put a .38 on layaway?

This person is a life long drunk and drug user, now in his late 60’s. He starts his day with a Bloody Mary, and drinks on through the day, turns to hard liquor in the evening.
He becomes belligerent and violent when he’s been drinking and using, which is a daily occurrence.
He lives in a very low income area prone to violence and break ins.

On FB he posted that he’d passed a background check and put a .38 on layaway because of the “kooks” in his neighborhood. He doesn’t realize, of course, that he’s the Head Kook Of All.

This is an absolute tragedy waiting to happen. What can I do?