
When you post a new status, there's a little menu that lets you prevent individuals from seeing it. Just to the left of the 'post' button. Click it and click 'more options', that'll expand the menu. Click 'custom', and under 'don't share this with', start typing your friend's name. It'll come up in a drop-down list, you can select it then click 'save changes'. Job done :-)
One thing though - check that the status stays as 'custom' each time you post; sometimes it does, sometimes it doesn't.