My friend has completed all of the missions as a host, but the only one that I still have not completed is Loose Cheng. Can my friend play all of the Casino Missions as a host after finishing all of them?
Yes, he should have the
Ab mission spot on the map within the casino to replay any mission after completing them all. No phone calls or waiting needed. They'll also get them as options when you do 'Replay Job'.
Once you've done them once, you won't get the first-time money rewards again though - host or not. You'll have to host them all in sequence to get the free car at the end.