Thank you for the opportunity to help you with your question!
the CPA firms can control the non-CPA's firms conducts by ensuring they meet the required standards by ensuring they follow the outlined standards and make sue they meet them fully in this case the CPA firms understands the conducts and other ethical measures which all users needs to meet in order to get the expected results
CPA member are required to make sure they report any irregularity shown so that they can get to help in making sure whatever they do is in compliance with the outlined measures
Please let me know if you need any clarification. I'm always happy to answer your questions.