. . . If this girl is under the legal age she can (I'm sure) get the police involved and have him arrested. Most countries have laws against blackmail and should protect her even if she is over the legal age of consent where ever she is.
Even if this guy did post his video wouldn't it only be on a porn site since things like YouTube are against that kind of content? And it would be statutory rape (depending on if the country this is from has such a law) and he would be faced with jail if she is under age.