Additionally often people like to know a link goes from A > B when they hover them and generally most scripts unless injected in will not do this, so as SEO stand point its not very good at all. Since Google believes visitors should see transparent links.
So in summary the 2 problems you face with type of linking is, Google may believe your trying to link to bad neighbors and cloak those links, or Google will not like it because your external links to your visitors are not visible. In my honest opinion if your client cares about SEO cloaking links should be avoided, it could lead to a kicking by Google.
A better way of linking out if he doesn't want to pass juice either in nofollow, or follow (Nofollow, still links some juice - reason I mention it) is to use a redirect script such as
domainname.com/redirect?http://www.another-domainname.com least that way the visitors see the end result - I know I'm repeating myself here but Google strong believes that users should be able to see where their going before they have gone :) So redirect? is meeting you half way but personally I'd just keep it normal ;)
Cloaking refers to the practice of presenting different content or URLs to human users and search engines.