There isn't just a 'dashboard_only' capability, or something similar, in Splunk.
If you really want a user to be able to only view one specific dashboard and nothing else, you'll want to create a new app and put that dashboard in it and restrict that user to have access to that app, and make sure the app permissions for that role are set to 'read'. Set the dashboard permissions to 'read' for that role. You'll also want to make sure the dashboard is the landing page/only page on the app, if that's all you want them to see.
here's a link for app dev.
http://dev.splunk.com/view/webframework-developapps/SP-CAAAEUC
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