I endorse the valuable opinion of Learned Umakanthan Sir. In HR circles, this is referred to as moonlighting, i.e., the act of working at an extra job, especially without telling your main employer. This is definitely an unhealthy practice and has to be dealt with strongly, as per the rules. If engaging in private trade or business is misconduct, then the employee can be booked under that. Otherwise, there may be a general clause that the employee is expected to devote full time and energy to the employer's business. If there is none, act fast and introduce one such rule. If the employee still persists in his second job, then action is to be taken.
Factories Act and Second Jobs
The Factories Act specifically prohibits a second job. Section 60 of The Factories Act, 1948, states: "Restriction on double employment. —No adult worker shall be required or allowed to work in any factory on any day on which he has already been working in any other factory, save in such circumstances as may be prescribed." A second job robs an employee of his vigor and enthusiasm as is required, and double botheration means the employee is unable to put in the required effort which one is capable of. Riding on two boats is always dangerous, as the saying goes.
Right to Form Trade Unions
The right to form a trade union or become a member or an office bearer is a fundamental right guaranteed under Article 19(1)(c) of the Constitution and hence it cannot be taken away.