I have purchased an mini ipad from a friend. It however, is locked. How can I unlock this?

You have encountered activation lock or perhaps the screen lock code. You need to go back to the gentlemen you purchased the device and get them to unlock it and turn off Find My Phone. If you cannot contact this gentlemen then you have been scammed and probably purchased a stolen device. You can get around the screen lock, you cannot get around the activation lock without the assistance of the person that turned on Find My Phone.