a link between 360 and one

You just need to sign into it on the One. https://support.xbox.com/en-US/xbox-one/console/sign-in-solution

That is my point: he can system link between two Ones or 360s but not between a 360 and a One. Maybe BC will fix it. Either way he still has to have 2 seperate copies of game...one for each console. He wants to play system link with just one game copy
and he can't. And of course the game has to have a system link option.

That link is for Xbox 360, you can not transfer digital content between Xbox One accounts. Still unsure as to what you are trying to achieve!

That is not what the op is asking. And no, you can't play a system link game between two seperate consoles without two seperate copies of the game. And right now since the backwards compatibility hasn't been fully implemented so you can't do a system link
between the One and 360 anyway.

Are you trying to system link between Xbox ones that are playing the Halo: Master Chief Collection and Xbox 360s that are playing the Xbox 360/Xbox Original versions of those games? I'm pretty sure that cross-play between Xbox One and Xbox 360 versions of
the games will not work. You can cross-play with the Xbox 360 version of the games on both consoles, for example, Halo 3, Halo 3: ODST and Halo 4 are backwards compatible on Xbox one.

You will either need all Xbox ones playing the Halo: Master Chief Collection or all Xbox 360s running the Xbox 360 Halo game of your choice.