How do I remove all photos from an iPhone I got from a friend?

Did you friend erase the iPhone, as described in this related article?

What to do before you sell, give away, or trade in your iPhone or iPad - Apple Support

After that you can set up the iPhone with your Apple Account and start fresh with iCloud Photos.

As Ingo2711 said, you should reset the phone-- not just for pictures. If you try to continue without resetting, you could find your pictures or messages and stuff going to your friend's phone, or your friend's stuff coming to yours! iPhones are meant to be private things!