To unlock a user in Linux is quite easy you just need to run the following command:
passwd -u <account>
This will unlock the account which has expired, but if you have turned on the advanced PAM Account Auditing you will also need to reset their account using the following command:
/sbin/pam_tally2 –user <account> –reset
This will reset the number of times they have incorrectly logged in so the account can be used again.