Whom Do I inform about a case of migrant worker exploitation ?

I have just witnessed A construction site manager threatening an agency worker to the extent that if the worker left to start a better paid job, that he the manager would sack the workers cousin and blacken both their names to agencies and also do his utmost to make things very difficult for the worker's new employer to gain any further work from the site mangers company. This goes on constantly in theConstruction Industry but I feel that I need to do something about this case of blatant oppression..... the worker himself came to the UK to escape this and finds that we are no different from his country... So what can I do about it?????