How do I block a friend on Facebook seeing my friends list?

[Shelley]

I've got friend on Facebook who won't let me see his/hers friends list. I want to do the same to him or her. Does anyone know how to do this?
im not sure how to do that. i dont want to block this friend, i want this friend not to see my friends list. how?

[Rock Lobster]

Set your friends list to be seen only by you or by categories of friends (Acquaintances, Family, etc) that do not include the friend you want to block.