Schedule a meeting with the professor, have all your evidence ready and links for him to check for himself, and present your findings in a professional and respectful way.
No way in the world he should get away with this, but going postal about it could end up hurting you later on. Do it in a way he doesn't know who turned him in and thinks the prof. figured it out on his own.
I've turned in a few people in my english classes and a couple of electronics classes for this. It's not right and the teachers want to know if it's happening. It benefits no one for them to get away with it, including them. Just be discreet about it. DO NOT confront them about it, that's like trying to discipline a co-worker. Go to their supervisor.