Eu tenho isso de uma fonte :
If I recall correctly, the "user name" is actually the name set in the Excel Options, rather than the actual user that's logged on.
As such, it could quite easily be any old rubbish.
Have you checked the open files on the share to find out if anyone really has it open?
Uma possível causa ( fonte ):
When Excel opens a file it creates a temporary copy of the file, and then as you work with it it makes changes to that file. When you save the file it then deletes the original, replaces with the temp file and delets the temp file. If the machine or app crashes before this happens you can get stuck with the temp file nor being deleted. Next time you open the file, Excel will attempt to create a temp file and realise that one is already there, hence the message that the file is already open.
Try deleting all files in the temp directories on both his machine and the server (may have to reboot).
Se você quiser abri-lo sem saber quem o tem, verifique estes métodos:
Go to Computer Management where Shared Folders may help. Open a run box, type Compmgmt.msc, then go to Shared Folders and look at Open Files.
Try the NET FILE command. Open a run box, type CMD to open a command prompt. At the prompt, type NET FILE. It will show any remote users with files open on the current machine.
If those 2 methods fail, go here: http://technet.microsoft.com/en-us/sysinternals/bb896655
This SysInternal tool will show you any process that is locking a file. This is your answer if the other two simpler methods dont turn up any leads.