The tough part of hiking this trail with someone less able is that the heavy-duty climbing starts right away, so you will need to hang back with her. If your group hikes on ahead, you might ask them to wait at the top of Nevada Falls, or you could try to catch up at the top of the HD.
Depending on your GF's stamina, she could wait (a very long time) at the top of Vernal Falls, or Nevada Falls. Either place, she could wander around and enjoy the scenery and people-watch. Maybe take a comfy hikers chair for her to relax in, and a book. (Locate the restrooms at both areas.) Or, she could proceed to wander down the JMT back to the valley floor on her own pace. There will be quite a few other hikers, so she would not be alone. Just make sure she knows all of the trails (Mist Trail, JMT, and the Clark Point connector in between the JMT and the top of Vernal, and the stock trail that goes to Happy Isles on the S side of the river.) If she has a little map, and can read it comfortably, she should be just fine. Make sure you have a plan on specific places to wait and meet, and an alternate plan. Cell phones work in some of the places -- Top of Vernal, HD, etc. (but my Sprint service did not work last week).
Now a tip for you: Once you get to the cables, if you are comfortable, it is easier to hike up the cables on the right (outside), because the granite is less polished from so many hikers, and your shoes will grip better. Only drawback is that the stopping/rest points (little ledges) are less frequent than the wooden cross-members between cables. Also (this is important), go up withOUT gloves -- your hands will grip the cable better. Go down WITH gloves, so you can let the cable slide through your hands. Leather gloves are best to allow cable to slip.
