There are 2 ways to allow your developer to have access to this page, one is
Step 1: Install User Role Editor Plugin and create a custom role for your developer and assign manage_options capability, but this will also allow him to have access to admin related menus like settings, etc, so to avoid that, install Adminimize Plugin and remove admin menus for that particular role
Step 2: Create a user with the new role you have created and share the credentials with your developer.
Another is, add a custom code in your theme / child theme (if child theme is active) which will allow you to set custom capability for the Discourse admin menu access, create a custom role using code and assign the new capability to the role and create a user with the custom role and share the credentials with your developer, but in this method your developer can only see the page but can’t edit/update/save any changes, since manage_options capability is not provided which is required to save options in WordPress. If you want I can add the code on github and share the link here.
Try any of the above and let me know,