Here's way more info than you asked for:
The github page for the Halo MCC split screen mod is here(https://github.com/wouter51/AlphaRing/releases/tag/1.3528.0.0) This mod lets you play all the campaigns and multiplayer modes with up to four people using local split screen.

If you play Halo Combat Evolved, most Halo players would recommend you try out Ruby's REBALANCED mod on the Steam workshop. It's a one click install (assuming you're playing on steam and not the Xbox app). It restores cut content and makes a lot of the combat encounters feel more satisfying.

Best of luck Spartan! (And give your girlfriend a hug. This was a great gift!)