I honestly don’t have a clue. You can use either an acf/pre_save_post filter or an acf/save_post action to do something after acf has saved the field values, but how to create a user that is only allowed to edit a post that was just created is not something I’ve seen and not something that ACF will do for you. Look into creating a user in code for WP and search for how to make it possible for them to only edit a single post.
Welcome to the Advanced Custom Fields community forum.
Browse through ideas, snippets of code, questions and answers between fellow ACF users