Microsoft doesn’t support PST files larger than 2GB, and they may become corrupt, so you will have to span the mailbox emails to several PST files. There are a few scripts out there that can do it, but I’ve never used them personally. To manually do it give yourself full access to their mailbox and map the mailbox in your outlook. Create a PST file and drag and drop the emails to the PST file until you get close to 2GB then create a new one.
There WAS a limitation of 2 GB archived files with Outlook 2000 and 2002(office xp), however when office 2003 was introduced the archived file became unicode and it does NOT have a 2GB limitation, it can be as large as 20GB now.