If she has never gone backpacking before, it should be one night to somewhere really nice.
If she additionally does not hike at all — it needs to be easy peasy and beautiful and one night, one night, one night.
If she’s just new to ultralight backpacking, and has good hiking legs and a spunky good outlook, you might try starting in Crescent Meadow and heading out along the High Sierra Trail — which for the first segment is not all that high, and while you would be camping at a creek in a flat spot at one of a few designated campsites along the way, there are great views from the trail as you go along, it starts in a sequoia grove, and that’s so early that it’s likely you won’t have a ton of competition from people wanting to do the entire trail — they’ll have to wait until the snow in the high country is melted. And the lower, parallel trail, on the middle fork trail — while not starting in big trees or having the excellent views — would be a decent fall back option. You can rent canisters in the visitor center at the Ash Mountain entrance. Two days out, two days back. Athletic hikers might make a loop of the two if they make it all the way to Redwood Meadow, where there is a trail junction.