Windows 7: Unable to see all machines on other domain with Windows 7

I have a single user who is logged in with a domain account on a domain that I don't maintain. On that domain they are able to see all local resources but not on my domain. I have made sure on my domain controller that the users account is part of the domain admin group, have forced a policy update to all of the machines in my domain and even added his PC as part of my domain. I have also verified that network discovery is turned on, IPV6 is disabled and that all security measures are turned off on his side. I can ping him from my domain by both his machine name and by IP but he is still unable to log in to certain machines on my domain. Does any one have any clue as to why this would be?

Thank you



Doug