Is your city relatively flat? SS sounds cool and fun until you have to go up a slope. As someone above mentioned, does she know how to use the gears of her current bike? This could be a factor.

I used to have a steel step through bike which was really heavy, and only had 6 speeds, then I switched to an aluminum hardtail MTB and even that is lighter so it allows me to ride faster. I think she'll notice a big difference with a nice aluminum frame and skinnier wheels, even with the 'added' weight of brakes and extra gears.