When viewing an article you have permission to edit, you see two tabs at the top - "Edit" and "Edit source". When viewing an article you don't have permission to edit, you see only one - "view source".
This is not good UI design There needs to always be the same number of tabs displayed. Ideally there should be tabs to "view source" and "view visual source" (with better wording though!), see Bug 51547.
If a read-only editor is not possible or not available then that tab needs to pop up a message saying that editing is restricted currently/until xx:xx for <log entry> and that they can view the wikitext source if they want (linked to an explanation what that means).