Trusted Domain Wireless Access

I have a Windows Server 2003 domain. I have successfully set up IAS with
EAP-TLS authentication and WPA encryption. This works great for members of
my domain. But I have users from another company that are set up with a 2 way
trust (not in the same forest) that want to connect wirelessly when they
visit our office. They do not have any Radius set up on their domain and it
is a Win 2000 AD domain. What is the best way to set them up. I would like
something better than WEP and MAC filtering if possible. Any help would be
appreciated!
-Steve