There shouldn't be any need to install anything yourself - in my experience Windows XP and Vista have all managed to install built-in drivers for mass storage device automatically when the phone is connected in that mode.
I only have experience with 32bit Windows versions though, are you using Vista 64bit by any chance?
Do you have a memory card in the phone?
Without one it's not a mass storage device!
I'm sure he does.
If he doesn't, the phone will refuse to connect in mass storage mode at all, and will just keep asking for a different mode to be selected - Vista won't even get to see the new device at all, let along give any errors.