Once a person is under ESI medial/sickness treatment , sec.73 of ESI act impose a bar on the employer not to terminate his service. Such termination would be illegal.
So, first of all , employer need to bring such person out of ESI sickness certificate, and for that you can contact the ESI doctor concerned, ask him the details of sickness and treatment provided by ESI and expected time of sickness. You can also mark such communication to Medical Director of the state. This will build pressure on ESI medical doctor. If the sickness certificate is being procured on false grounds, in all probabilities, doctor will stop issuing further certificate and give him the fitness certificate.If the sickness is genuine, you need to wait.
Once ESI issues him fitness certificate, and even then he do not join, then he can be put to disciplinary proceedings for unauthorised absence.
regds