How to restore or hard reset an iPhone 6?

A couple weeks back my best friend lost her iPhone in my house. We looked everywhere & ended up giving up. She went to sprint told them she didn't know what happened to it (unsure if it was even reported stolen) & bought a new phone. We have now found the phone but is useless to her since she got a new one. Is it still able to be used? & how to hard reset it, she doesn't even remember the password used on the phone.

Before she does any of that, she'll have to take it back to Sprint where they can un-blacklist it. Once a phone is blacklisted (reported lost or stolen) the serial number is blacklisted in the national database. It can never be reactivated in the US ever again. They have to do that part first before you bother with the other stuff. Best luck.