Start here:
"If you forgot your Apple ID password" click here --> https://support.apple.com/HT201487
What to do after [or before] you change your Apple ID email address or password - https://support.apple.com/HT204071
I don't want to bewilder you with a lot of links at the start (possibly others will differ and do so, but that's up to them), so see if that works.
Unfortunately if you have set "Find My" on the phone (a default when setting up I believe) you cannot erase the phone and make it usable. It will demand the old Apple ID because it will think you are a thief trying to clear it for resale. Removing Activation Lock (= Find My) without the password cannot be done by a user and would require a trip to an Apple Store with the original receipt presented by the purchaser of the device. Even that is not guaranteed since this is not an official service offered by Apple. So it is best if he can recover his Apple ID password.