Honestly, I think you should inform your boss of the dangerous mistakes that Dave’s been making and force him to force Dave to either get help or put him on a leave of absence until he gets help. It’s not your responsibility to talk to his wife and quite honestly; she lives with him and must know how he is. She should know even better than you, and if she hasn’t forced him to get help by now then she either can’t or won’t.

If your boss won’t force Dave to do anything, I’d call OSHA. My Father worked in the construction field for years and I know that any small mistake can mean disaster, so I would demand that something be done.