Login to the server and (as root or the account owner) run 'mutt -f /path/to/his/mailbox'. From there you can type a capital 'T' (for "Tag messages matching the pattern I'm about to enter"), you will then be prompted to enter a pattern to match. Type '.*' (to match all strings) and enter. All messages should have an asterisk ("*") next to them. 

Now type a semicolon (";"--the semicolon tells mutt that the action you're about to take applies to all tagged messages) and a lowercase 'b' (for "bounce". Together ';b' means "bounce all these messages I've just tagged"). 

You'll be prompted for an address to bounce the messages to. Type it in and wait a little bit for all the messages to bounce (you may be prompted by mutt to confirm you want to bounce the messages). 

Type a 'q' to quit mutt.