Probation Period Extension from Backend after expiry

The managers have closed the probation period before the due date considering the performance of the employee. After few weeks the employee was forced to go on medical leave and his immediate manager was apprised of the situation. He is having the doctor’s certificate wherein complete rest was advised as his job involved travelling for more than 70-80 kms every day. His manager on daily basis was forcing him to resign and not avail the leaves. One fine day a mail was sent by his super manager to HR asking to extend the probation period ( of course from back end) and the reason cited was absence. On confronting them, they are now telling that his performance is not upto the expected level and he needs to have more system knowledge. But they have not put anything on mail. The employee has sent the mail that’s their conduct is not ethical and it amounts to manipulation of the system using the power they possess. The HR only responded that he received the mail to extend the probation period and he did it. I need to know how can they be tackled since they, including HR are not responding to the mails now. Is there any forum wherein this case can be taken as it’s related to a very well to do Insurance company.