I want to show/hide a custom field in a relationship form (for an intermediary post type in many-to-many relationship) based on the selection of another custom field in the same setting. [cred_show_group] does not seem to work and is displaying the shortcode as text on the front end.
When editing a Relationship Form with the visual editor you'll notice that the section to add a conditionally-displayed group which is normally available (see screenshot) is missing: there is no support for conditional field display in relationship forms.
That won't help much right now, though. If you want such functionality you would need to build it yourself with some JavaScript/jQuery.
Use the DOM ready event to hide the section you want to conditionally display.
Add an event listener to detect changes in the input for the field which would trigger the display and use it to toggle the visibility of the hidden section.