Where was the data initially stored? On Exchange or on a pst that was on his hard disk? If it was on Exchange, you should have made an Exchange backup first, and if it was all in a single pst, you should have copied it first.
You should have told him there's nothing to fix, he needs to delete his own stuff. Them empty the deleted items if it is indeed all trash in there. You touched it, now you're kind of stuck.
Outlook and Outlook Express do not handle large size files well, period. Your user wants stable e-mail, then he needs to store it on a mail server such as Exchange, which is designed to do that, or on a more expensive or robust mail client (something like Goldmine).
Assuming the mail was originally stored on Exchange, did you try removing the Exchange Service and all ties to the .pst file, then recreate the Exchange service? Basically, the default mail delivery location should be to the Exchange mailbox. If the data is gone off Exchange, you're pretty much screwed. You are either restoring from a backup (which if you don't have or didn't do, you're really screwed). Your only other decent option might be to run scanpst.exe or some other pst repair utility on the pst. What is the size of the pst you created? Does it look like it contains all the e-mail, if not, you did something wrong, or there was just too much mail for outlook. Also maybe search the hard disk for *.pst, maybe its in another pst file.
Matt J.
Please always take the time to backup any and all data before performing any actions suggested for ANY problem, regardless of how minor a change it might seem. Also test the backup to make sure it is intact.