Try TVS scotty electric
Is has a powerful motor of 800w which can go up to speeds like 50kmph which are enough for most girls.
Plus no petrol bike, makes it easy on pocket money.
If i was a Gal, i would have brought a Electric vehicle
*www.scootygals.com/teenzelectric.aspx
But yeah, she take a test drive before your buy.
see which one suits her the most.