At that point it's an archeological site, not an inbox. I'd almost hate to delete stuff pre-2000 just because it's a bit of a time capsule into a different era.
If you create a rule for a specific email address, you can send it all into a folder, then mark the folder as read. Or, you know, just send all to junk
If she read an email every 15 seconds on average to determine whether it needs to be kept or deleted, it would take her 34 days NON STOP to get through that list of unreads.... gotta just nuke and start over