playing halo on other accounts

So I downloaded Halo (download code) on my account thinking that it wouldn’t be a problem for my gf to play on her xbox account, but its saying that she has to buy another copy of the game to play.
Im just wondering if this is across the board and 343 did this for licensing reasons or if this is a problem that can be fixed?

IF you both have your xbox set to your home xbox, you should be able to share games
Check that, see if it works for you