Modifying Relationship.tpl modal
You can use preProcess or BuildForm hook on formname CustomDataType and unset the custom group or field based on user role or id

You can use PageRun hook and unset the variable in smarty template based on condition and it will not display on view page

since the custom fields are build using ajax, you can also use ajaxSuccess jq event to remove the custom field using Relationship.extra.tpl