Do the devs even assist with this problem my friend has made his post about not getting the halo 3 LASO achievement over a week ago and is now having even more MCC achievement issues. Devs are you fixing this?
Did your friend complete the playlist out of order? Maybe he joined you at Sierra 117 after you watched the Arrival cut scene by yourself or something? This is the most common issue with playlist achievements, happened to me on Halo 2 LASO. If that is the issue, I posted what I found about that here; https://www.halowaypoint.com/en-us/forums/84ad72a8b51847978545f685f651fc15/topics/psa-playlists-on-co-op-laso/b43747ff-e644-47fd-b4c5-6d8685111d95/posts?page=1#post5
Long story short, he just has to beat the last level again. If he isn't up to playing the whole playlist again, I plan on keeping a last mission save for all of the LASO playlists when I get to them on my playthroughs, and I will be able to pass that on to him. It wont be any time real soon unfortunately, due to heavy travel for work.