What did you mean by "recognized union"?
Recognition and Registration are two separate matters.
Recognition can be under a law (like in maharashtra, there is a law called, Maharashtra Recognition of Trade Unions and Prevention of Unfair Labour Practices Act) OR recognition may be accorded by the employer.
If you are dealing with a statutorily recognized union and the employee in question is NOT a member of such a union, then included under the "Rights of Unrecognized Unions" under the same act, even an unrecognized union may represent its members in disciplinary actions.
Replying or not replying a union is a different matter. Not replying a union does not help you resolve any issue, as even after an action is taken the aggreived employee is empowered by the Industrial Dispute Act to approach the labour machinery of the state like conciliation and later adjudication under Sec. 2-A of the act, when no union may sponsor his dispute!
In such matters it better seek answers by providing all related facts of the case or the advice you may receive may not be accurate!
Cheers!