Tell him your the FBI 
As much fun as that may be, he might be the type of guy to turn you in for impersonating a Federal agent. Also, tampering with his website probably isn't a very good idea.
Your best bet may be to contact the copyright owners of the ROMs and let them know what he is doing, he'd receive a cease and desist letter pretty quickly. The only issue with that is if they go to all the trouble of trying to take him down, it might bring unwanted action against the MAME community and some of our other backup ROM sources.
It's kind of a catch-22. One thing may be to go directly to the ISP who is hosting the account and let them know what he is doing may be illegal and that they could be named in any sort of lawsuit. That may at the very least get his account disable temporarily.
Please, don't do anything illegal to the guy or you're just stooping to his level and asking for trouble.