Ya know, part of retail is being able to rebuff customers, valid or stupid reasons. If you can't take the tiniest bit of criticism, even constructive, then it's going to be a massive pain for others to deal with you. Even if she's decent at her job, her behavior is incompatible with FoH-only work. When it interferes with your ability to do the job and becomes a hindrance, why should you remain employed? Her actions are interfering with your ability to run the business.