Yeah this'd be a very quick way of suddenly finding yourself uninvited from my apocalypse games. Apocalypse is about the spectacle, not rules lawyering. If the person/people on the forces of order side are already being this difficult about army selection and trying to impose their will on what their opponent can/can't play with then they'll probably be an absolute nightmare once the game starts. Expect hours of nitpicking arguing how the apocalypse rules interact with 7th.
Actually unless i'm missing something and they've changed the rules (I play 6th ed apocalypse) page 18 of the apocalypse rulebook pretty much blows their argument out of the water.
RAW
pg 18 Apocalypse rulebook "Armies may include any number of units from any codex, plus any number of fortifications and apocalypse formations. No force org restrictions apply to the army. In addition ignore unit restrictions that are normally limited 0-1 or that say you must field one of x to take one of y.."
pg 19 Apocalypse rulebook "Remember you can bring along units from more than one codex."
Heck there's nothing to stop your CSM player from fielding say a helldrake and then running the marines using loyalist rules.
