Dear friend,
Yourself being already a HR, I need not dwell much on the concept of dual employment.
Normally employers do not permit dual employment by their full time employees for the reasons of unified focus on their jobs and confidentiality of their own business interests.
Though it may not be a specific clause in the contract of employment, it is implied that a full-time regular employee should not simultaneously serve another employer of the same industry or otherwise.
Therefore, my suggestion to you would be to stop it forthwith.