What is the hard drive capacity and available free space?
All account information is stored in the Mail.app preference file which is located at Home > Library > Preferences > com.apple.mail.plist.
Not having adequate free hard drive space can cause the apparent loss or corruption of application preference files and although I've never experienced the same with any Mail.app version, the preference file can become corrupt which is why this file and all other application preference files should be included with a regular backup.
Did your friend upgrade to Tiger from Jaguar or Panther on this Mac?
Since the preference file data was lost or the file has some corruption, this will require re-entering all email account information but if your friend maintains a backup that includes Mail's preference file, this won't be required.
With the Mail.app quit and using the Finder, go to Home > Library > Mail. Copy the Mail folder and place the copy on the Desktop for backup purposes.
Using the Finder, go to Home > Library > Preferences > com.apple.mail.plist. Delete the com.apple.mail.plist file and empty the Trash.
This will require re-entering all email account information when re-launching Mail.
Launch Mail and decline any prompt to import mailboxes. After re-entering all email account information, the Mail.app should rediscover the existing account named folders and mailboxes at Home > Library > Mail.
After confirming all mailboxes were successfully rediscovered, you can delete the copy of the Mail folder from the Desktop or leave the copy there for the next steps required to resolve the download error message.
