You do not need to access the cpanel. That would be illegal. If you truly do need help via legal avenues to fight a defamation site, then contact me via PM, as this is something I've actually helped a number of people with in very legal ways. You say you've tried the "cops" - well, which ones? There are agencies that handle this, there are also lawyers.
And indeed CPanel is often a paid service on a dedicated or vps hosting solution, and there is a reason for that. It's good. Honestly, outside of lucky guesses at passwords, CPanel itself is pretty damn secure so... good luck with that. There are some things you can do if the host uses CPanel and WHM and it's a large shared hosting service concerning site mapping and possibly finding attack vectors through other websites hosted on the same server...... but I digress, chances are you are trying to do something illegal and your girlfriend story is just that... a story.
HackThisSite.org does not condone any illegal activity.