First, check to see at the login screen if there's a text box with the old domain, and a dropdown menu that allows you to select the local computer instead. If that's there, attempt to log on locally, using the same user name and password.
If that doesn't work, and presuming that he owns the laptop (it sounds this way, but you wouldn't necessarily want to mess with a corporate computer), you'll probably want to try something like resetting the local administrator password with a SAM file editor. Try
this for a summary, and
this for the
de facto standard tool for doing it.
Now, before you go doing that, ask whether NTFS encryption (the built-in encryption scheme in Windows, a.k.a. EFS) was not used on anything important. If it was used, you'll find that the encryption actually works, and you'll lose the data protected with it.
Also, if there's anything remotely important on that computer, you're going to want to consider making a backup image (i.e. copy of the data structure, not just the data) of the hard drive. That way, if you screw up, you won't lose it all. You'll of course need some way of reading that drive on another computer, and a way to run the imaging software. Maxtor MaxBlast is free, and works if you plan to plug it into a desktop that already contains a Maxtor, Seagate, or maybe Quantum hard drive. (Acronis TrueImage is the commercial version of that software that doesn't check to see if a particular brand of hard drive is installed.) Symantec Ghost works well for this too, especially the older versions.