Although I think Renji did begin to realise the full deepness of the situation in the previous episodes, still this must have been a very bad shock to him. One might say almost as bad as it was to Chihiro herself, who, at the very least, doesn't remember what she lost even if she had to face such an oblivion the first thing in the morning. I wonder if Renji has what it takes to start everything from the beginning with her - knowing the same might happen again one day. Although who knows if it's even possible. Now that Chihiro doesn't remember Renji anymore at all, she might feel uncomfortable reading the diary yet not feeling anything.