If they are both under 18 then they can BOTH get in trouble. If her bf wants to get revenge do whats best under 18. Beat the ex's ***. No, im not kidding. Becasue if the police where to get involved she would probably get in as much trouble by her parents as the ex would by law. Also, by the time he turns 18 his juvenile record would be erased so it wouldnt matter anyways. Its equaly as much her fault for doing it, as it is his fault for being an immature *** and uploading it to Youtube. So her current bf should be questioning why shes doing that at 15 anyways (or younger since you didnt mention when these actualy happend) as much as trying to get revenge on the ex. Under these circumstances the current bf may be ****ed but its really not any of his business.
Another thing, if they where to actualy bring in the police i bet some Drs will be brought up and if the current bf has had any current sexual relations with his gf then he will be brought into court as well for having sex with a minor. When the law comes in, there are MANY things to take into account for. Been there, done that, 18+ ftw.