Just so you know VS As you know

the first is providing information to someone

the second is confirming that the other person has the information already

__

they are quite different

Right. "Just so you know. . .", means you are informing someone of something.

"As you know. . ." is really stating your assumption of another's knowledge.

They are not quite the same.

"Just so you know" is mostly used to warn or alert the listener, and carries the implied message "Don't say later on that you weren't told".

"Just so you know, smoking is not allowed"
"Just so you know, the last bus leaves at 8 PM"

Let me get this straight.

The phrase 'just so you know' implies which of these two?

1. You already know that, but I'm telling you that again.
2. You don't know that so I'm letting you know that.

I'm having an argument with a friend of mine,
and he insists that 1 is more like the phrase than 2.

Would you please help us find out who is right?

Your friend is just wrong. Number two is correct.