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I don't think she would go wrong with a Lumina 900. I honestly think it is a pretty sweet piece of equipment. It may not be absolutely cutting edge, but it is not too far from it. It has a smooth OS, a single core 1.4Ghz processor and 4G LTE so it is fast, and a 1830mAh battery so it should have decent battery life. The display is big and bright with reasonably high resolution. I haven't owned a Nokia device for a while, but one thing that I did experience with all the Nokia devices I have owned through the years was an above average build quality.
On the downside, she would have far few apps than she would on either the App Store or Android Market aka Google Play. Still there are 80,000 plus apps available for it, so it should take a while to go through all of those.
I will not buy a Windows Phone because I've used Android devices for quite a while and am familiar with the OS and the apps. These are the same reasons that I won't be likely to buy an iPhone, which I also think is an excellent piece of hardware. If I was new to smartphones like your coworker, I would view the Lumina 900 as an attractive choice at an attractive price. When you amortize purchase price over 2 years, even an extra $200 up front only works out to $8.33 per month, so I would not let the on-contract purchase price sway me too much.