I've got this from a source:
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?
A possible cause (source):
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).
If you want to open it without knowing who has it, check these methods:
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
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.