My understanding is that the passes now allow in/out access that day at any lounge at the airport (subject to capacity restrictions). Best to confirm with agent before doing so, but that means she can stop in for a coffee, restroom, etc after her (presumed) redeye, then head out into the city for several hours, and come back and use the lounge again before the evening flight.

As others suggested, 12 hours in any of the lounges would be quite dull and I would advise against, but if she's going to do it, I suppose I'd choose the C18 lounge. The crowds will swell several times during the day, but it is the nicest one. If she wants less crowded, then B6.