Facebook friends

There are a few kinda, sorta ways of doing this. The truth is that FB privacy settings are unnecessarily convoluted, and they often don't work as expected.

Let's use names here as it's easier. Let's say you want to be friends with Joe and Mary, but you don't want Joe to know about Mary. Hence the scenario is unilateral (e. g. it only goes in one direction and you don't care if Mary knows about Joe).

Two options.
1 - Friend them both but put them into separate lists. Share with one list or the other. This doesn't hide so much as it obfuscates. Joe will still be able to discover your relationship with Mary. All you are doing is making it less obvious. It's a lousy solution.

2 - Make a hidden, secret group and invited Mary to it instead of friending her on FB. You will probably need to have her email address for this. You may or may not be able to find her on FB even if you aren't friended; some of that has to do with the other relationships you both have, including mutual friends, if any. But an email should work.

This is a far better solution although it is a pain in the ass to maintain. You will have to remember to share with Mary's secret group in addition to your wall, that sort of thing. It is not impossible and, personally, I run a bunch of groups and it's fine. But recognize that it is more work. Add as many people to the Mary group as you like, but understand that you'll need to swear them all to secrecy about your relationship with Mary.

Now, if it's bilateral, and you don't want Mary to know about Joe either then, you guessed it, you'll need two separate hidden, secret groups, and you won't friend either of them. Again, if more members are added to the secret groups, then you will need to keep a leash on those folks, and that's difficult if not impossible to do.

Like I said, Facebook can be more complicated than it needs to be, and this is stuff for you to have to keep straight. Every single day. Difficult but not impossible, but it can be a lot of work for what really should be a leisure activity.