I bought my brothers old iPhone 6 Plus from him, factory reset it & all but for some reason the standard mail app is still on his

I bought my brothers old iPhone 6 Plus from him, factory reset it & all but for some reason the standard mail app is still on his

Android would of worked

You can just sign out of his account, although a factory reset should've done this. Are you sure you did a full reset? Go to Settings > General > Reset and choose Erase all content & settings. When it finishes, be sure to set up as new device