It wouldn't surprise Arthur if Charlie had something worse to add to his experience than he can imagine. And even as exhausted as he is, there's a part of his mind calculating: if he does give Charlie the out, would they ever get the opportunity to talk about it again? As horrible as the experiences were, at least he had someone who knew what it was like on his side of the bars.

"Yes, I- um. I... sort of assumed from it that you had... someone like Faust, a-as well."

There's not really a good way to say 'hey chum we sure have both eaten human flesh huh?'.