This is XP Pro?
Have you right clicked the shared drive to make sure all permissions there where set?
May have to look under administrative tools, Local security policy for changes or the error log on his/her computer even yours. This is all under the control panel.
Could also have a some registry corruption. Has the clients and/or yours been reset sense you last re-added the user? Don't know if you leave them on all the time or not. Just wondering if things would have been updated like they should have when you made these changes.
When this person tries to open the document, does it say any error? Or it just can't be found?