[bulbasnore]

grep'd the insert commands for the numeric userid out of the backups of

user
userfield
usertextfield

If you don't know how to do that, without further explanation, you are in danger of wrecking your database, so instead make a new user though admincp for this person.

Either way, at this point you now have a new user with a numeric userid, so follow this query:

UPDATE post SET userid='newusersid' WHERE username='exusername';

newuserid is the numeric userid of the user
exusername is the EXACT string for the deleted user's username