Theft is never good, but such "soft" theft is all too common these days. Downloads of music, movies, using other people's artwork and writing, stealing bandwidth, and more.
First of all, forgive them. Love them -- not their action.
If there is some way to contact them, do so, and explain your concern. If they take down their crime and apologize, then great. Your done.
If they are recalcitrant in their attitude and persist in their crime, then report them to the service they are using. If they are using a free online service, like Google, then tell that service. If it is their own website, then find out the service which hosts the website through WHOIS or similar service, and write the web hosting company that one of their customers is involved in illegal activity.
In some instances, there may be nothing you can do. The criminal may be hosting the website on their own equipment and in a country which has no laws concerning plaigerism. But if you truly forgive, then you will be free of the burden of resentment. And you will have done everything physically possible to resolve the crime, and you can move on with your life.