I'm guessing 1616's, but you should get a couple of 1516's, 1616's and 1716's to see which fly best out of her particular bow. If she's at all interested, she'll break or lose the mis-matched ones pretty soon anyway, and the upside is you'll get better matched arrows in the long run that way.
Most arrows in that spine use the 50 grain glue in points. These are pretty light points, and if you cut the shafts to 27" for her, the arrows could well turn out too stiff. It's better to stick with full length shafts and the light weight points, because the overall arrow weight will be lower that way than if you used heavier points and a shorter shaft, and if she's only pulling 21#, she's going to be much happier with lightweight arrows.