How to remove Apple id from iPhone 6 that has been factory reset (but the Apple id is still on there)?

Brother of my girlfriend got his iPhone 6 back from an acquaintance, and wants to use it since his phone croaked. Problem is that the other guy set it up with his apple ID, and we have no access to that. So basically everything asked for that login and can't be used. I being their resident IT guy searched on Google and found out that you can factory reset the iPhone with iTunes. Some sites say that it also resets the apple ID, an I believed it. Well, it doesn't, so now even the basic stuff can't be used since it asks for an apple ID login right at startup. How can this be removed/bypassed? Phone can't be booted properly, since on startup it asks for Activation lock sign in, with apple ID and password. If there is no way to crack it, I don't mind advising them to get the other guy to unlock it/give password (as far as I understand this is not really an option, but it's not like I care), but I'm a PC/android user, so I would very much appreciate a hacky solution instead :)