Hi George,
Maybe post what SAP MII Roles is assigned to the user.
Check the Roles if it included the XMII_Developer Role and that his role has the
Action XMII_Developer included.
Or Try a test and create a new Role add to this role the Action : XMII_Workbench_all
See if this works
Otherwise delete the user and Re-create the user by using the Copy To New user Action from another developers user that can access the workbench
