Dear Khaleel,
I invite your kind attention to section 22 of the existing Trade Unions Act, 1926, which is still in force, and section 23 of the Code on Industrial Relations, 2020, which is going to replace the former upon its notification for enforcement. Both of these sections deal with the proportion of the number of outsiders being the office bearers of a trade union. As per the explanations provided under both sections, a retired or retrenched employee shall not be construed as an outsider for the purpose of holding an office in a trade union.
For argument's sake, if it is held that a dismissed worker cannot bear the status of a retired or retrenched employee, subject to the condition of such persons' number remaining within the proportion mentioned in the respective sections, they have to be treated as outsiders only.
Therefore, in my opinion, any dismissed workman can be an office bearer of a trade union functioning in his erstwhile employer's industry subject to the proportionality of the outsider representation mentioned in the above laws as applicable.