Imagine that I have two friends - Alice and Bob - and I want to prevent Alice from showing up in Bob's friend suggestions ("people you might know"). Can I do that?
You cannot do that directly, but you can ask Alice to block Bob and then Bob will not see her.
To block Bob, Alice can go to Bob's profile and click "Report/Block this Person" and check the "Block this Person" box, or she can go to Account » Privacy Settings » Block Lists » Block Users and enter Bob's name or email address.
You will need to tell Alice the following:
Choose the "Privacy Settings" option in the Account drop-down menu at the top of any Facebook page. Select "View settings" in the "Basic Directory Information" section. Change the setting for "Send me friend requests" to "Friends of Friends"