How can I prevent my friend's cousin from remotely accessing their iPhone?

My friend's iPhone was set up by his cousin when he bought it for the first time. Now, his cousin can see his Instagram, Snapchat and everything on the phone. He even tells him what he has been searching for. I think he might have installed something on the iPhone but never found it. My friend wants to get rid of this issue so that his cousin can no longer access his iPhone. I need a solution to fix this problem.

Should I reinstall iOS using iTunes? Should I change the Apple ID? Or is there a loophole in the phone that I should check? Please provide a solution for this.

[Re-Titled by Moderator]