Dear Puja,
The ultimate and the twin objective of punishing employees on the ground of indiscipline is reformation of the erring employees and the deterrence against such tendency in others. Therefore, punishment should be such that it should punish the misconduct only and not the individual. In other words, the nature of the punishment should be proportionate to the gravity of the charges proved.
Dismissal of an employee on disciplinary grounds, whether he is permanent or temporary, automatically becomes a stigma on his character and affects his employability in future. Since it is an economic death sentence affecting not only the individual but also his family, revocation or modification of such an extreme punishment is an issue based on the facts of every such case. For obvious reasons, your post lacks the complete facts of the case. Therefore, the pressure applied by the trade union alone can not be a consideration for reversal of a dismissal. At times, the unduly long period of temporary employment and the associated ill treatment meted out to such employees by supervisors/ middle level managers can be a reason for the negative behaviour of the individuals leading to unpleasant disciplinary action and the ultimate punishment of dismissal.
Therefore, let your management have an unbiased introspection of the entire episode leading to dismissal and come to a decision independent of the pressure applied by the individual through the trade union.