The Toshiba Chromebook 2 is widely regarded as one of the best--it's one of the faster ones, and has one of the best screens. I would also recommend the HP 14--I have the previous version with the Haswell processor, and it's excellent.
If she wants a smaller (and cheaper) one, go for the Acer CB3. It's the successor to the C720, and from what I could tell when checking it out in Best Buy, they improved the screen, the keyboard, and the speakers. Processor should be the same, if not better, in performance. It's a great deal at $199, sometimes even less when on sale.