That user/workstation is probably using <i>Exchange cached mode</i>.
In this mode, there is no .pst file. Instead, Outlook uses an .ost file usualy placed in C:\Documents and Settings\username\Local Settings\Application Data\Microsoft\Outlook\outlook.ost
In this case, my advice is telling you to “export” this users mailbox.
You can do it easily in Outlook -> File -> Import and Export -> Select Export to a file -> Personal folders file (.pst) then select the root of the mailbox and put a checkmark in Include subfolders. Wait until it ends. Depending on the mailbox size, this might take several minutes.
Once the machine is formated, invert the process, i.e. import the exported file.