There isn't any reason you can't hang out at the United Lounge if you can't beg your way inot the Delta one, it's over by T-14, as far as you can get from where you need to be, but you have plenty of time.

Check in with her, hang out in the lounge, see her to the gate, back to the lounge, then get your plane.

If I remember, the RCC in Atlanta isn't all that much of a lounge, but still better then the gates.