None of them would joke about something like this, but it's true that Enjolras hardly jokes in general.

He meets Courfeyrac's eyes with his own rare, bright smile.

"Come in quietly. He's already asleep."

Of course Courfeyrac will want to see. And Combeferre is very likely too soundly asleep already to wake even for a friend's arrival, to judge by how instantly sleep claimed him; but, if not, he'll want to wake and see Courfeyrac bright-eyed and whole. There's no reason at all for them to whisper out in a hallway.