About Security regarding installation of 3rd party apps

I have known for a very long time that apple does not recommend installing 3rd party apps in their phones. However, what if someone silly like my friend made a mistake and installed a 3rd party app that asked for a configuration profile (said verified btw) onto their iPhone.. What can the app do at most?.. what do the profile allow it to access?(bank details? his photos? accounts?) How can i- i mean my friend secure his phone afterwards? or is it compromised forever? or is it even compromised?(am i overthinking?) how would i know?