Because Outlook for the Mac is an OS 9 only application that will probably not be updated to run nativly in OS X. He could sync the folder that contains all the information manually, but you runt he risk of accidently deleting or reverting to an earlier time this way.

Also, forgot to say this in the last message. Exchange does not work for Entourage beyond mail, and only when Exchange has IMAP enabled. Probably better to go with a free IMAP server that your father can connect to.