If the union is supporting, instigating any act whci could be described as unfair labour practice, process for cancellation of its registration/ recognition can always be initiated.
A registered trade union can have up to 25% of its office bearers as outsiders, i.e those who are not working in the industry. Accordingly, a person who is an outsider can be an office bearer.
However, if he is a Gorernment employee, he is obliged to take permission of his employer for engaging himself in the Union activities of other companies. In your case, if the Union leader is an employee working in the Government organisation, you can write to his employer and seek the clarification. You can also write to the organisation and inform that he is participating in the Union activities in your organisation, while he is supposed to be on duty at his own place of work. This is with a view to put some pressure on the organisation where he is working. However, if he is left free by his organisation, you have no other option but to deal with him as a Union Office bearer.