How do I change the registered owner of an iPhone?

Get your friend to do all these things first or ask for your money back. They have otherwise sold you a useless device. What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

After that see Set up your iPhone or iPad - Apple Support

Your friend needed to do what is detailed in this link before you can use the phone --> What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support