Rights question

Hi,
Forgive me if this is a dumb question, but the concept of blogging is new to me. I am setting the blog up for a friend who’s website I maintain. I tried setting up a category for him that allows him to post, but not for anyone to respond (There is another category for that), but he is unable to post into that category. He has full Admin rights, so that part is covered.
I thought (perhaps mistakenly) that as long as he had administrative rights, he could post into any category regardless of who set it up. Am I wrong? How can I set this up so he can post, or does he have to set up the category himself? (He’s not all that PC-literate).

Thanks in advance,
Jake