If you have about 6,000 yen you could get a day-plan at the Narita Rest House (It's on the airport campus). But really, you're still stuck in Narita airport if you stay there.
I would say that it's best to spend 500yen for the roundtrip to Narita city on the Keisei line. That way you can at least kill a few hours walking around the temple complex.
I don't see any "day rate" plans at the cheaper hotels in Narita City - most of them state 3pm as check in time. (It might be worth calling to see if you can get a day-plan).
After going back to the airport, rather than renting a day-room, perhaps you could see if a FlyerTalker will be able to invite her into one of the club rooms.
I hope this helps.