can someone help me with a facebook question?

[pri✍]

No, you can't make just one friend (A) that do not know that you're friend with somebody (B), especially if they're friends to each other.

If they do not know each other (mutual friend), you can prevent EVERY friends of yours to know whoever your friends are by setting your Friends list privacy to Only Me, ie. by click the Friends list in your Timeline ➤ click pencil icon on top right ➤ click Edit Privacy ➤ click Only Me.

But, it's BUT, that friend A,eventually know that you know B, if
• you written B name in any of your post or comment, even they don't friends to each other;
• you comments/likes B's post;
• you tag B;
visa versa, etc. , especially in public post.