The contact details are stored in a file of USERNAME.wab where USERNAME is the user name (believe it or not).
If you open Address Book and click Help and then About Address Book, it will tell you where this file is located.
If you use Windows Explorer to navigate to this location, you may find an old version of the file there, called USERNAME.wab~
If there is such a file, make sure OE and Address Book are closed, then rename USERNAME.wab to, say, USERNAMEXXX.wab and then rename USERNAME.wab~ to USERNAME.wab
Then open Address Book and see whether it improves the situation (make sure that nothing else is missing). If it doesn't improve the situation, suggest you reverse the renames and just accept that the contact details are gone (unless your friend has some other sort of backup, which sounds unlikely).
Late night thought, and longhand solution, but if my pal has not deleted his old sent/received emails, how about listing them by received/sent-to alphabetically in columns and opening them once by contact name, right clicking on contact name in the 'sent to' or 'received from' box and using the option of Save to Address Book? I think that works in Out. Xpress.