Need to show disclaimer when user leaves site

I have a client that needs to show users a disclaimer when they leave the site so they know they are leaving and the site owner isn’t responsible for the views of external sites, etc, etc.

I’ve searched around and did not see any posts or plugins that helped me. I know there is code for various php and js scripts that will do it different ways. Before I start wrecking things, does anyone have any suggestions and/or know of a plugin that will do this.