It is good decission that you want to quit your job immediately,when things are not good, are to be left instantly to get mental solace.
If you are thinking the idea of quiting is best for you. Did the employees raise any alarm against the working for 12 hours ? If no, then you are not the right person to raise the matter on behalf of the workmen rather your role is a communicator to the management on the basis of complaint received from the employees. Are you also working for 12 hours? If yes, what you have done against the extra hour of working without substantial payment as laid down in stattute.
Is your boss propritor of the firm or is working for someone? You know the answer well and this will clarify your doubts. This kind of situation is prevaling everywhere, how many job you will take-up and leave.
As an HR you can do a lot against the odd, if there is a wish. If any employee comes to you with complaint against 'hour of working'. You can handle the employee by saying, though this is the decission of the management still you will put the matter before management. You can tell them that they can can knock the door of labour department, if they feel something wrong is happening.