If it were me, I would discuss your feelings ~ together~ then postpone the wedding ~together. Clearly he doesn't see the gravity of this, or that it is deceptive. Not even admitting to the clear evidence is .... bold, to say the least.
Wouldn't you rather have a your beloved treating you, talking to you,(& only you) the way he is with the "old friend"? I would.

Wouldn't you rather be planning your wedding together? Planning your future, ... um, together?

He isn't ready for marriage. Just my opinion. I would take a break & put this whole thing on hold. Maybe he'll realize this is more than just a little faux pas.

It is way easier to clear the air now than in divorce court. You really don't want to go there...

Good Luck in this. My heart goes out to you ....