that moment is before everything sarah, right? geee, i don't know. well, the most obvious thing is that he was protective but it's kinda without sense, greg's a friend. so maybe he showed his dissaproval to Greg's immature behavior, and his look says "I would never do THAT with a pillow". or he thinks that after ND, after HE found out about sara's past and greg doesn't know, he is supposed to defend her from jokes like that, from people who doesn't know although don't mean to say something wrong. maybe he was just making sure greg won't go too far with that.