If he is using the x64 version of Vista then it won't work. No idea how to get it to work either, just about all versions on Gunz I have tried on my computer using XP Pro x64 none of them will load.
Most likely driver issues, driver support on x64 machines is pretty bad right now and quite a few games won't run on these operating systems.
If your user is simply using standard x86 (32 Bit) version then he has corrupt files or some other unknown problem because it works for me.