How do I block a friend on Facebook from seeing what I post on another friend's timeline?

[pri✍]

If you post on your friend's Timeline, who see that post is determined by that friend, not you. Think logically.
Same thing, it's you who control who can see any post in your Timeline, not others. Isn't that what you want?

What do actually you want?
Do you want the post to be seen by that friend only?
Do you want the post can be seen by all your friend but one person?

Whatever it's, you have to do that in your Timeline or Share to your own Timeline; there you can change the audience setting according to your wish who may see it.
Don't forget, the gem of the choices, the Custom, as in there you can determine who can and who cannot see it.