You're right to doubt your friend on this for several reasons:
* The Limewire Pro software is a ripoff, as you get the same access with Limewire.
* Most P2P networks and software are crawling with malware.
* No matter whether you purchase the software or not does not matter. The question is whether or not the content is legal. Well over 90% of the files available on P2P networks is pirated, and thus illegal to upload, download, and use no matter what. Thus, paying a couple of bucks for the software does not make downloading music from 50Cent, Britney Spears, etc legal. You can still be tracked down and sued as it's still piracy.
To read up on this issue, click here for a previous thread on the topic. Specifically mentioned in some of the posts is Limewire Pro, the laws, reasons for P2P, and more.
Additional things I'd like to mention:
* The US government recently ruled that the owners and operators of P2P networks are responsible for everything that is 'shared.' Thus, they are being forced to either close down (which a lot of them are, including grokster), go legit (like Napster did in the past, and iMesh did recently), or face lawsuits from the RIAA, MPAA, and individual developers, producers, etc.
* France is currently debating (to my knowledge they have not reached a final decision yet) whether or not to legalize P2P networks. If they do, you'll have to pay a nominal monthly fee for unlimited use that would include everything shared on any/all networks. Aside from that, there are laws forbidding piracy, including 'sharing' copyrighted materials over P2P networks, in almost all modern countries.
Once you've read the previous discussion and gotten a handle on it, make sure you relay the information to your friend, who's either in the dark on the issue, believing what others may have told him, or in denial over the legalities and security issues involved.
Hope this helps,