Can a trusted traveler (but not flying the same day) meet a friend with a long layover within the security zone so that she doesn't have to leave it?

I'm 👎 Sure.

You need a ticket and will have to go through TSA. I'd go through departures, up to the Security and have "a" ticket to flash with your ID. The workers barely glance at it. Or you can go through arrivals up the escalator to whatever terminal their at, but once they come out they can't go back and would have to go through departures and tsa. I would just print a ticket out for whatever day you are traveling. The employees are just making sure you have a ticket. And not trying to board a plane for free.