Does the bike have a single chainring, or a double or triple?

You would want a rear wheel made for single speed, they often come with a back pedal brake built in, which can be handy if she likes it. Otherwise you would need a free-wheel.

I rode a singlespeed bike for about 5 years in town.
And maybe because of that I just don't get the recent hype around singlespeed/fixed gear bikes. With a single speed you either never can ride as fast as you like (because your gear is not high enough) or you can't get over steep inclines without fearing to damage your chain/chainring - or, most often, both, depending on the situation. And I'm not talking about long steep ascents, only about every day bike use in a not too hilly city.
I don't see why a singlespeed would be an improvement over the use of at least some gears in any possible way, really.