No correct information sure is provided on the main profile page.
But I suppose it is not a pedophile case you have here, perhaps a iGangster instead. Although chances of him using a proxy is small.
You should also get his IP address and some other stuff that is stored on the Bebo database.
One thing that will work for Bebo sure is threatening lawsuit which will also include Bebo as a testifier or something, I don't know. Stuff like that gets people moving, even if they're probably 2000 miles away.
Bebo servers are hosted in California, United States. I'm quite sure that in US (huge rate of online criminal activities / bullying), there are laws that also make internet harassment illegal. Basically the action took place in USA. When this might go into something huge, Bebo sure will be providing logs and sure will your ISP - or maybe not (in Estonia there's a law that went live on 1st April that makes all ISPs keep the packet logs that go through). That all if it is too hard to prove harassment is taking place by looking at your sister's chat logs.
Another way to get this guyse IP address is to add this freak on MSN, close all other applications that use the internet (so we could identify the harasser's IP address). Then you (or your sister) has to fool the guy into receiving a file. Make it called something like "nude_pics" or whatever you'll call it. It can be in any format. Perhaps image format (.png, .jpg, .jpeg) will suit the best, but WARNING: MSN has a thumbnail display of the image, too. So you better actually make it contain something nude or seducive (check out the sexy girls topic on TMS for example). Press the "start" bar, select "run", write "cmd" and when file is transferring, type in "netstat" to see all connections. Or for better saving and overviewing, use
WireShark to trace. Let me remind you that all packet/connection logs play a big role if this case is going to go big. When you have the guy's IP address, you can further act with authorities.
How the law works depends on where you live. Some "development" countries (as they're called?) don't even have internet laws yet - you can also take it to the USA court then as Bebo (or where harassment went on) is located there.
I see many female people having trouble with so-called "pervert" there, so it is better if you'd feed packet/connection logs or IP address to your authorities, otherwise the investigation most likely will be stuck. Government doesn't usually have such section for internet crimes, so there won't be real investigation. All that will happen is that you receive a letter in a year that they have been unable to find the pervert. That's why you should act first, collect a lot of data on your or Bebo side - with packet logs, connection logs, chat logs, IP addresses, etc. to make the case roll.
I hope you bother reading it all. Also, you should try to find someone else (female I suppose) who the dude has been harassing, so you could "team up". So the main message is not to break up "relations" with this "pervert", just cause you could collect more information (even PRIVATE INFORMATION).
But if this "pervert" turns out to be some weird guy that goes on 4chan and doesn't give a fuck (means he's not really a pervert or something), it will be a little harder to build a case, but try to follow my instruction (advice), team up with others (if you don't get what I mean here, get someone else like your parents to read it or something), and do your best!
oh. wait, I forgot to say that I'm here for advice (techniques to chase this guy, legal help).