You can see the D5100 vs the A57 here
http://snapsort.com/compare/Nikon-D5100-vs-Sony-SLT-A57
They both have their advantages and disadvantages. Look through that site and see which features each one has and then see what she needs.
I've got to say that after having tried a few Canons (1100D, 550D, 600D, 650D), I wasn't very impressed. I much preferred the feel of the Nikons. I felt they were better built too. I've tried 3 Nikons, the D3100, D3200 and D5100 and all were great.
Really though, as I said before, don't just buy one online for her hoping she'll like it. Take her to a shop and get her to try out a few. It's all very well buying the D5100 over the A57 for example because specs-wise it's possibly better, but at the end of the day, if she doesn't like the menus or the ergonomics or general how it feels, she's not going to like the camera.