OE crawls with more than a few hundred emails per folder or db file and has the same size limit.
You can try what Les suggested or back up the db file and rename it to something like Backup2 in the same "hidden" folder and teach this person to purge the sent and inbox and dleted folders below 100 files but not delete from server, but performance will suffer everytime a sync operation is done on startup.
They can also use the online webmail to fetch mail as a backup which can be configured to use any IMAP or POP server.
OE is a very lite mail client, never intended for 2GB of storage or over hundreds of emails per folder. Creating new folders can make this run faster and use sort, drag N drop or delete as it is a very inefficient database compared to Outlook.