As I said, if a doctor says he is unfit to work that means what it says!
He is UNFIT to work! What he feels is not relevant. If he did work against medical advice the companys insurance will not cover him if he gets injured. So the company can not allow him to work as he would not be insured.
If as you say he was made to lift heavy items on his own then that will have contributed to the injury so the company can be liable for his injury.
He should get medical advice about the possibility of suing the company for damages. Once he starts a case against the company they can not dismiss him while the case is ongoing. This is almost exactly what happened to my son. He was told to lift a heavy item at work and injured his shoulder doing it, it should have been a two man job. He was off 7 months but he kept his job and they had to pay him £5,000 compensation!