Kindly look into the standing orders applicable to the establishment and extended to the designation of employee, appointment letter, service rules and regulation and check if it mentioned that employee during employment can't enter into gainful employment/association/venture elsewhere............................
If it is stated then it is doubtful that the employee shall succeed.
However it shall be appropriate to show all the docs to a competent and experienced labor consultant/service lawyer and proceed under expert advice of the lawyer in this matter.
The lawyer that has examined the docs and merits shall be best placed to advise in such a matter.
Valuable advise of learned experts/members is sought.