Rekordbox license key on used DDJ-400

Hi all,

I'm looking into buying a DDJ-400 and having some issues understanding how the licensing system works exactly.

My friend bought a brand new DDJ-400 and has gotten a license along with it that activates the Core version of Rekordbox 6, lifetime, for free.

I am also however reading that when you purchase a secondhand version of the DDJ-4, you don't necessarily need the previous owner's license key because it's hardware unlocked(?). I currently have my eye on one particular listing, but it comes without a license key.

Am I understanding correctly I can still use rekordbox, based on his license even if I don't get the key from him? And will it be a problem if he has a license for version 5, and I want to use version 6? And will that be for a Core version as well, is that a default for all licenses?

There probably is a very clear explanation, but I've kind of lost track of how it all works haha.