I believe that "arrested" and "un-arrested" are most commonly used as adjectives describing the state of the individual and "to de-arrest" is a (clumsy) verb describing what is done to him.
The waters are muddied by our wonderful language which has the past participle "arrested" as a transitive verb as well as an adjective.
On balance, although I don't like it, 'de-arrested' is probably correct.