To remove a friend from a certain category:

1. go to the "Friends" section;
2. on the left-hand side of the page choose the category from which you would like to remove your friend;
3. hold the cursor over the picture of the user you would like to remove from that category;
4. click "Delete from the list" in the menu that comes up.

The friend you select will be deleted from this category but remain on your general friend list as part of the "All" category.