Company President is friends with volatile Director

I am an HR Manager
· President of company is good friends with a Director
· Received feedback that this Director is communicating abrasively to his staff, has drastic mood swings and a volatile temper
· Attempts to address the Director’s behavior resulted in numerous “I’ll handle it” discussions with the President
· No improvement seen by HR… His staff is losing sleep, becoming emotional and stressed
· President has refused to allow the situation to be addressed by HR
What steps should I take to resolve this situation?