Unfortunately we currently do not have a way to programmatically interact with the 'Required' form field attribute. One option could be to create a second required comment field that is hidden by default and invoke a JavaScript function onChange of the roles field to show/hide the necessary comment field. Note that as Catalog does not store the state of form fields governed via JavaScript you will need to create a function invoked onLoad of the form to also check and show/hide the appropriate field.