One option is to find out what their userid was and insert a record for
them back into the profiles table in MySQL. Very much a back-door
operation, and may not work correctly, so be careful.
Another option is to create a new account for the user, and update each
field in each table that references the old account:
update bugs set reporter=<newid> where reporter=<oldid>
update bugs set qa_contact=<newid> where qa_contact=<oldid>
update bugs set assigned_to=<newid> where assigned_to=<oldid>
update bugs_activity set who=<newid> where who=<oldid>
There are a ton more places where that needs to be updated, but sanity
check can point those out for you.