This to confirm that the user is able to run a search using this index but not through a dashboard.
Is the other user able to use the index by issuing the search command index=yourindexname
?
You have to set the indices which are used by default via "Settings => Access controls => Roles => RoleName => Indexes searched by default"
You can specify which indexes are searchable by default to users via user roles and rights, based on user or group. See the docs :
http://docs.splunk.com/Documentation/Splunk/latest/Admin/Aboutusersandroles