I doubt there is a case. Once he put the video into the public domain it was fair game.
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Depending on the specific facts, CNN might have a fair use argument.
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He could but he would Cee Nexta Nuttin out of suing CNN except for attribution and some very costly publicity from the suit itself. Reason? CNN was, from your description, using it for NEWS [ as in their middle name] and COMMENTARY and CRITICISM which are the epitome of fair use and free speech, and this would be the classic case for which fair use and free speech were intended by Congress and the Constitution. Seamus sounds like he might be an infringer himself, if he is posting large portions of video games, so I doubt he about to take on those huge lawyers. You might not be doing him any favors. He would be much better off just insisting that CNN provide the attribution that is required under 17 USC 106A since his is an audiovisual work. Now if they did not give the attribution that might make a "Federal case."
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First, it sounds like your buddy Seamus may be violating copyright law by playing video games and recording commentary on You Tube, He therefore has "unclean hands" which would make it very difficult and risky for him to bring suit. Further, CNN is probably saved by the fair use doctrine (although it should have provided attribution). In addition, if your friend did not register his You Tube videos with the copyright office before CNN used them, he may be prevented from suing CNN---registration is a pre-requisite for a law suit under copyright law. Most importantly, it is hard to see how your friend suffered any real damage here----this case is probably going to cost more to litigate than could be recovered. You are not doing your friend any favors here by suggesting that he sue, but if you and he want to look at this issue you should retain IP counsel. And my guess is that no lawyer will do this for free--be prepared to invest some significant money into an investigation by IP counsel if you want to pursue this.