Has she ever ridden a fixed gear bike? If not, she may want to start with a flip-flop rear hub. That is a rear wheel that is fixed on one side like a track bike and a single speed on the other that will coast. So you can turn the wheel over and use whichever you want. I have a bike with one and switch back and forth depending on what I'm doing. At least she would have a choice if she doesn't like the fixed gear.
With all the people building fixed gear bikes to sell, make sure the one you get has sufficient adjustment to tension the chain properly. If you are unsure what you are looking for, I suggest going to your local bike shop and talking it over with them.