How do play friends maps (ninja warrior) in halo 5

It sounds like he's trying to play a custom game within Halo 5 that's titled Ninja Warrior.
If that's the case, he'll have to search in the custom games for the map name, download it, and invite his friends to play.

Hey Jenine

Thank you for taking the time to post this on the Xbox Forums. We understand that you are currently inquiring about certain maps on the title Halo 5.

I would have to refer you to the post made by Xbox Forums Ambassador PWNRR, as this is how you can find this content.

For these types of questions, I would recommend that you visit the
Halo Waypoint for everything Halo related.

If you are inquiring specific help related to this game, that would be your best bet, as they also have a forums section that would be able to much more properly cater to you.

I hope this was of some use to you

^Rage Warrior All the PC maps from from Halo CE, Halo 2 Vista will be included in the Master Collection. It has been confirmed from 343i, that in fact all of the Maps that have been released from all four games will in fact be playable on Xbox One.

^xCHEMISTx Agreed.

The Halo 5 Beta starts on December 27th, 2014

1) Dynasty Warriors 6, 7 and 8

2) Samurai Warriors 2

3) Ninja Gaiden 2 and 3

Update! I got it. This method works! Keep doing it every time you play Halo 5. Make sure the games are complete games that start at the 0 second marker. Patience do it a little bit every day. Re-synching and play a few maps, get your wins and play with
friends with mics.