If she is young and athletic, she will want to more than just hike to Vernal Falls. I recommend hiking the Mist Trail up alongside Vernal Falls to the top, then crossing on the bridge to continue to the top of Nevada Falls. Return by way of the John Muir Trail. This has been one of my favorite Yosemite hikes for the 50+ years I have been going to Yosemite, starting with 1969 when I lived and worked in Yosemite Valley.
Will she be going alone, and will she have a car? It is pretty much essential, especially if she is staying outside the park at Yosemite Riverside Inn, which is near Groveland. And if she is going solo, it is still OK ago hike; I did lots of hiking alone when I was living there. There are many bothers on the trails so one is never really “alone”.
If she is indeed staying near Groveland, she will have to queue at the Big Oak Flat entrance station each morning. The line can be long—-1-2 hours, even in September. We waited for 45 minutes to get through that entrance last year in mid-October, because we joined the queue at 9:30 am. Best to get a much earlier start.
With 3 nights and 2full days, I recommend spending one day in Yosemite Valley and one driving the Tioga Road east to Tuolumne Meadows to see the peaks and high meadows. That is actually my favorite part of Yosemite in summer, as the Valley is very crowded. Great stops along the road are Olmsted Point, a quick stop with parking on the right (going east). This is the view from there:
https://www.yosemitehikes.com/tioga-road/olmstead-point/olmstead-point.htm
Next stop could be beautiful Lake Tenaya. There are parking areas at both ends of the lake. We prefer the western end, for the view in this classic Ansel Adams photo:
https://shop.anseladams.com/products/lake-tenaya-1?variant=31415638622275
There is also good swimming there. The eastern end has a larger parking area, Sandy beach, and picnic tables and a restroom. One can walk the lakeshore trail on the far side (away from the busy road) as far as one likes from either end.
https://www.yosemitehikes.com/tioga-road/tenaya-lake/tenaya-lake.htm
Then continue to Tuolumne Meadows. For a fantastic viewpoint, and another of our favorite “hikes”, park at the obvious parking area on the left just as one enters the meadows, and walk up Pothole Dome:
https://www.yosemitehikes.com/tioga-road/pothole-dome/pothole-dome.htm
For the easiest walk up the smooth sloping dome, follow the obvious trail that starts to the left of the parking area as you face the dome. Follow it in the horseshoe skirting the meadow (do not even think about treading on the meadow itself) and alongside the steep sides of the dome. Continue to the eastern most side which has the most gentle slop and just walk up where you like ( best not to be wearing smooth-soled shoes).
We like to do this late in the day, around 5 pm, so she may want to do this after visiting the meadow itself. There is a visitor center with parking on the right, a mile or more east of the dome. It was not open when we were there last October but it should be this year. If not, the map they give out at the entrance station will show the meadow trails one can wander. The Soda Springs marks the site of the old Sierra Club Campground (now obliterated) and there is a day lodge nearby (Parsons Lodge), which occasionally has a volunteer on site to talk to.
https://www.yosemite.com/things-to-do/adventure-activities/soda-springs-parsons-lodge/
This itinerary, with one day for the Valley and one for Tioga Road, omits the Mariposa Grove of giant sequoias, but it is an hour’s drive from Yosemite Valley, on the opposite side from the Tioga road, so it would take half a day to get there and back. If she wants to see sequoias there is a small grove at the start of the Tioga road. It is a one-mile walk from the parking area to the trees.
https://www.yosemitehikes.com/tioga-road/pothole-dome/pothole-dome.htm