Whether to tell his prospective employer that a new male post doc opposes affirmative action for women in academia?
Mind your own business.

If you believe that your colleague poses an actual threat to someone, it is of course your duty to warn them.  But opinions are not threats.  It is entirely up to your colleague who he shares his opinions with.  Revealing your colleague's opinions to his future employer, no matter how offensive you may find them, would be a violation of his privacy.