He cannot log into his old domain unless there is a trust between your domain and the old domain.
He can access the old domain after logging in with his current information. It is Cross Domain Authentication.
You should be able to access the old domain by authenticating with:
<pre>Domain\Username</pre> of the old domain + password for that domain.
You can also change the password on a second domain at the CNTL – ALT – DEL | Change Password by specifying the other domain (manually type it in).