Where it came from doesn't matter - talking about raping a little girl is despicable and horrible to hear no matter who it's from. When it's your own daughter? ...Fuck.
Bottom line: He needs to apologize. Sincerely. In more than two words.
I don't think rex ever did go that far, though. I know there wasn't anything sexual in the posts before the raid. If anything sexual did come up with that alt, that sounds more like Pariah's style than rex's.