Unfortunatly your friend was alone and there was a group of frat boys it boils down to his word against a group of cowardly liars.
With the amount of surveillance in the world today, the odds of this happening where it was caught on tape is probably pretty good. Especially in a college town.
College towns are in fact actual towns. They usually have an actual police department that has nothing to do with the college cops. They are almost always at odds with each other. Whichever one gave you the brush off, go to the other and play both sides against the middle. You can't guarantee a good investigation, but you should be able to get a case opened.
If the locals aren't responsive, you can try the state police as well.
Additionally, you can pursue it within the school's disciplinary system. IF they weren't just frat boy types, but anyone was wearing letters, you can take it up with the internal system that deals with frats and granting them charters on campus.
Then of course there is bad press and negative attention. You can bitch and moan about assaults not being followed up on to the mayor's office or the town council. Someone is getting paid so they don' t have to hear about this crap, and they aren't doing their job. Additionally, you can bring it to the local press. Also, don't rule out the student newspaper. Tuition goes up every year, and it isn't that hard to get the student paper to ask just what the heck are the students paying for. Being ignored by emergency services is usually not considered amongst the things one shells out good money for. Bad press in the student paper gets more attention then you would think from the administration.